To guide you through the world of web performance, we share our own definitions of technical jargon here in French. They differ from what you find in most online glossaries and lexicons for a good reason: we aimed to make them as understandable as possible for beginners. Happy reading!
A
A/B Testing
Tool for comparing the effectiveness of several versions of the same web page on real users, particularly regarding conversion rates. Its technical complexity has led to the emergence of specialized providers with paid solutions: AB Tasty, Kameleoon… On the web performance side, these solutions are worth monitoring: their scripts, often blocking to avoid any "flicker" effect (fleeting display of the original version), can significantly delay page rendering.
Accessibility
Refers to the ability of a website to be optimally usable by all users regardless of their browser, location, or physical and psychological abilities. The WCAG guidelines, which are the reference, focus on people with disabilities and seniors, who often experience difficulties navigating the internet.
Ajax
Acronym for “Asynchronous JavaScript and XML,” it is a technology that allows a web browser to exchange data with a server without having to reload the current page. It is the basis for tools like Infinite Scroll, as well as SPAs that function like an application.
API
Acronym for “Application Programming Interface,” it refers to the ability for one application to grant access to all or part of its functionalities to a third-party application. The goal is to facilitate its use and integration into an existing workflow. For example, you watch series on your smart TV thanks to Netflix's API.
APM
Acronym for “Application Performance Management,” it refers to a tool installed on a web server to monitor its performance. To do this, it connects to the key components of the operating system (processor, RAM, system kernel, etc.) and specific applications (SQL databases, web server, PHP executable, cache system, etc.).
ARIA
Acronym for "Accessible Rich Internet Applications," refers to markup intended to add an additional semantic layer to web pages. By supplementing HTML tags with specific attributes, ARIA informs browsers and especially assistive technologies (screen readers in particular) about the role and expected behavior of each interface element.
Asynchronous
As opposed to synchronous, asynchronous loading occurs in the background, allowing the rest of the page to continue loading without interruption.
B
Back-end
Part of a site accessible only to the site's owners and managers, as opposed to the front-end, which is accessible to all internet users. This concerns the administration and all scripts and tools executed by the web server.
Base64
Encoding system that allows data, often binary, to be represented as text composed of 64 ASCII characters. Encoded content is on average 33% larger. On the web, it is mainly used to embed resources inline (images, fonts…).
bfcache
Contraction of "back/forward cache." A browser mechanism that keeps a complete copy of visited pages in memory: when a visitor uses the back/forward buttons, the page reloads instantly, without any reloading. Certain technical practices (HTTP headers, scripts…) can deprive a site of this valuable mechanism.
Blocking Time
Period during which a web page is no longer interactive due to excessive device resource usage by JavaScript. This results in freezes and/or excessive latency.
Brotli
Modern compression algorithm used by web servers to reduce the size of text-encoded resources (HTML, CSS, JavaScript…), and thus their download time by browsers. Newer than Gzip, it offers a better compression ratio. Its compression is relatively slow, making it particularly suitable for static resources (CSS, JavaScript, fonts…), which can be compressed once and for all, and less so for dynamic content.
C
Cache
Principle of keeping a ready-to-use copy of content that is expensive to produce, in order to serve it faster on subsequent occasions. It is found at all levels of the web: browser cache (which avoids re-downloading resources), HTML page cache on the server side (which avoids regenerating each page on each visit), object cache like Redis (which relieves the database), or CDN cache. It is one of the most powerful levers for web performance, with TTFB at the forefront.
CDN
Acronym for "Content Delivery Network", this tool interfaces between hosting and visitor browsers to improve site loading times. It relies in particular on copying static resources from the origin server to each data center of the provider worldwide. Cloudflare is the most popular. More info.
Critical Rendering Path
Translation of "Critical Rendering Path": the set of steps and resources essential for the browser to display the first screen of a page (in particular, blocking HTML, CSS, and JavaScript). The shorter and lighter this path, the faster the page appears (FCP, LCP). Optimizing it involves reducing, lightening, or deferring everything that is not essential for this first display.
CLS
Acronym for “Cumulative Layout Shift,” this performance indicator measures a page’s ability to provide an interface whose elements remain visually stable over time. It is one of the 3 Core Web Vitals. A good CLS is below 0.1, a bad one above 0.25.
CMP
Acronym for “Consent Management Platform,” which translates to French as “plateforme de gestion du consentement.” It is the tool that allows collecting visitor consent during their first visit regarding tracking and cookie placement. It is an essential tool for GDPR compliance on any site that processes personal data. Feel free to consult our CMP performance comparison.
CMS
Acronym for “Content Management System,” it is a tool that allows a website and its content to be managed through an online administration interface. The best known is WordPress, but there are hundreds, sometimes specialized in certain types of content, like PrestaShop and Magento for e-commerce.
Concatenation
Optimization technique for loading times aimed at grouping several CSS or JavaScript files into one. Common for reducing the number of HTTP requests for a page, this technique has however lost some of its relevance since HTTP/2, which multiplexes requests: it can even harm the granularity of caching. Synonym: combination.
Core Web Vitals
Core Web Vitals, or Signaux Web Essentiels in French, group 3 metrics intended to measure a page’s performance in terms of speed and responsiveness: LCP, INP, and CLS, with INP having replaced FID in March 2024. They are part of the Web Vitals, which include other measures like FCP or TTFB. These are the performance indicators taken into account by Google in its algorithm and tools: PageSpeed Insights, Lighthouse, etc.
Crawl budget
Refers to the volume of resources Google allocates to crawling a site: number of pages crawled and frequency of visits by its indexing bots. The faster a site responds (TTFB in particular), the more effectively this budget is used: more pages are explored, and therefore indexed. In French, we speak of “budget d’exploration.”
CrUX
Acronym for “Chrome User Experience Report,” this tool developed by Google allows you to know the real performance level of a page or a site. To do this, it collects performance indicators, including Core Web Vitals, during visitor navigation on browsers that support it. The field data thus collected is accessible via Search Console and in the “Discover your users’ experience” section of PageSpeed Insights reports.
CSP
Acronym for "Content Security Policy", which translates to "politique de sécurité des contenus" in French. It is an HTTP header that aims to indicate to the browser the authorized origins for different types of resources: CSS, JavaScript, images, iframes, etc. Implementing a CSP improves a site's security but also allows for an inventory to be taken, which is very useful for web performance.
CSR
Acronym for “Client Side Rendering,” which translates to “rendu côté client” in French. In the SPA ecosystem, this means that the JavaScript framework is responsible for rendering the components of a web application in the browser. This practice has a major impact on performance, both in terms of speed and responsiveness. The alternative technique is SSR.
CSS
Acronym for “Cascading Style Sheets,” which translates to French as “feuilles de style en cascade.” It is the language used by web browsers to format pages in HTML and XML: display of text, position and color of elements, animations… It is overseen by the W3C.
D
DNS
Acronym for “Domain Name System,” this protocol is used by browsers to direct visitors of a page to the appropriate server. It associates each domain name with an IP address using data provided by its owner (DNS records). A global cache infrastructure ensures its responsiveness, but in return generates propagation delays in case of server changes.
DOM
The DOM (Document Object Model) is the browser's representation of a page's structure, with each HTML tag becoming a "node". It evolves dynamically based on node additions, deletions, and modifications made in JavaScript.
E
Embed
Translates to "embedded content" in English, and materializes within a web page by including content from a third-party site. Social networks like Instagram, X (formerly Twitter), or YouTube all offer this type of integration.
F
Fallback
A fallback can be translated into French as "solution de repli" or "à défaut de". On the web, it consists of offering an alternative in case a feature or resource format is not supported by the browser.
FCP
Acronym for "First Contentful Paint", this performance indicator measures the time it takes for the first visible element of the page to appear (image, text, etc.). It is heavily impacted by any resources that block page rendering (scripts and stylesheets).
Fetchpriority
HTML attribute allowing to over-prioritize (value "high") or de-prioritize (value "low") the loading of resources in the waterfall. It is the most effective way to modify the browser's default behavior, to be preferred over preloads. Applied to an image that constitutes the LCP, it improves the metric by 15% on average.
FID
Acronym for "First Input Delay", this performance indicator measured the delay between a user's first interaction (click, typing, etc.) and the start of its processing by the browser. It was part of the Core Web Vitals until the introduction of INP in March 2024. A good FID was under 100 ms, a bad one above 300 ms.
Breadcrumb
Visual representation of the current page's position within a site. Each element of the hierarchy is clickable to allow a quick return to the homepage and parent pages (category for an article, for example). In English, we speak of "breadcrumb".
Front-end
Part of a website accessible to all internet users, as opposed to the back-end which is only accessible to the site's owners and managers.
FTP
Acronym for "File Transfer Protocol", this protocol allows data to be transferred to a web server's storage space: images, PHP files, etc. It is only accessible to the site owner through login credentials. It is now largely replaced by its secure variant SFTP, which encrypts exchanges via SSH.
Full stack
Refers to a developer who masters server-side technical environments (PHP language, SQL syntax, system administration, etc.) and browser-side environments (HTML, CSS, JavaScript, etc.). These profiles are rare and sought after by many companies for their great versatility and their ability to work on both the front-end and the back-end.
G
Gzip
Compression algorithm historically used by web servers to reduce the size of text-based resources (HTML, CSS, JavaScript, etc.), and thus their download time by browsers.
H
.htaccess
System file consulted by Apache and LiteSpeed web servers before serving a page or resource. It is commonly used to implement URL rewriting, manage 301 redirects, add security headers, or optimize browser caching of resources.
HTML
Acronym for "Hypertext Markup Language", it is the language at the base of any web page. It consists of multiple tags, each with its own role: paragraphs, images, tables, sections... Attributes allow developers to modify their behavior as needed.
HTTP
Acronym for "Hypertext Transfer Protocol", it is the exchange language between browsers and web servers: the former requests a resource, the latter delivers it. Its version 2 (2015) introduced request multiplexing, i.e., the ability to process several requests simultaneously over the same connection. Version 3, based on the QUIC protocol, further speeds up connection establishment, especially on mobile networks.
Hydration
Stage of a Single Page Application (SPA) loading during which JavaScript makes the components already displayed on the page interactive, usually after server-side rendering (SSR). CPU-intensive, it is one of the main performance bottlenecks for web applications, particularly INP.
I
Icon Font
Set of icons grouped within a Webfont in the form of characters. The format is very popular because it offers vector rendering and is easy to use, but it requires a significant volume of CSS and Webfonts in return. The most popular is Font Awesome.
Iframe
HTML element allowing a web page to be displayed within another web page in a space with defined dimensions. It is the preferred solution for displaying most advertising banners and social media embeds.
Infinite Scrolling
UI element alternative to traditional multi-page pagination. With "infinite scrolling", the next contents of the list are dynamically loaded into the current page as it scrolls. Ajax technology is used for this.
Inline
So-called "inline" code is inserted directly within the original page, as opposed to a call in an external file. In HTML, the most common inline tags are for CSS, for JavaScript, or for images. It is also possible to use base64 encoding to integrate any resource inline.
INP
Acronym for "Interaction to Next Paint", this performance indicator measures a page's responsiveness following a user action such as a click, touch, or keyboard input. It was integrated into the Core Web Vitals in March 2024 to provide a more comprehensive view of latencies, particularly those related to JavaScript. A good INP should be less than 200 ms, and it is considered bad above 500 ms.
J
JavaScript
Programming language used by website creators to make their pages more dynamic, add an interactivity layer, display advertisements, or track their traffic. Code interpretation is resource-intensive for browsers: it often generates "Blocking Time" and degrades INP.
JSON
Acronym for "JavaScript Object Notation", it refers to a syntax for exchanging data between multiple tools. Its great simplicity facilitates its generation and reading by computer languages, including PHP and JavaScript. On the Internet, it is widely used by APIs and tools for website publishers.
Is your site as fast as your visitors expect?
L
Lazy-loading
Lazy-loading refers to triggering the loading of certain page resources (images, videos, embedded content, etc.) only when visitors are likely to see them. It is implemented using JavaScript tools or the native HTML attribute "loading=lazy".
Lazy-rendering
Technique that allows excluding part of the DOM from the initial rendering of a page using the CSS property "content-visibility". Rendering times are thus reduced, ensuring gains in speed (FCP, LCP, etc.) and potentially interactivity (reduction of Blocking Time). The only constraint is being able to anticipate the height of the excluded elements.
LCP
Acronym for "Largest Contentful Paint", this performance indicator measures the time it takes for the main visible element of the page to load. It is one of the 3 Core Web Vitals. A good LCP is under 2.5 seconds, a bad one above 4 seconds.
Lighthouse
Web audit tool developed by Google's teams, focusing on 4 aspects: Performance, Best Practices, Accessibility, and SEO. Any page can be tested to get its score (from 0 to 100), on desktop and mobile. The latter mode simulates degraded network and processing power conditions: scores are generally lower.
Fold line
The lower limit of the visible area of a page on the screen without scrolling ("above the fold" in English, a legacy from the print press). Everything displayed above it must be loaded first - this is generally where the LCP element is located - while what is below can be deferred, notably via lazy-loading.
Long Tasks
Translated into French as "long tasks", they refer to JavaScript executions that monopolize the processor for more than 50 milliseconds at a time. During this time, the browser cannot respond to any interaction: the page appears frozen. Identifying and splitting them is a major lever for improving INP.
M
Media Queries
Media Queries are a CSS tool that allows targeting specific devices and browsers based on their characteristics. They are used notably to create "Responsive Web Design" sites based on screen sizes, but also to detect the type of pointer (mouse, touchscreen, etc.) or to display a specific interface when dark mode is enabled.
Microdata
Refers to markup intended to present page data in a structured way. It can be integrated directly into HTML tags (microdata format) or via a script in JSON-LD format, recommended by Google. Google relies heavily on microdata to generate the "Rich Snippets" displayed on its results pages (star ratings, recipes, step-by-step guides, etc.). Synonyms: structured data, microdata.
Minification
Optimization technique aimed at reducing the size of CSS, JavaScript, and HTML files by removing all non-essential characters: spaces, line breaks, comments, optional syntax elements, etc.
Mojibake
Display of unreadable characters (a series of incoherent symbols like "é" instead of "é") that occurs when text is decoded with a character encoding different from the one used to save it, typically UTF-8 content interpreted as ISO-8859-1. The term comes from Japanese and literally means "character transformation".
O
Link obfuscation
SEO optimization technique aimed at hiding a hyperlink from indexing robots while ensuring transparent operation in browsers. This is usually done using JavaScript and base64 encoding.
OCSP Stapling
Translated as "OCSP Stapling" in French, this is a protocol that adds supplementary data during an HTTPS exchange between a server and a browser. This data allows the browser to verify the validity of the SSL certificate without having to query the issuing authority itself. As this check can delay the connection establishment, avoiding it improves TTFB. Note: the OCSP protocol is being gradually phased out in favor of Certificate Revocation Lists (CRLs), with Let's Encrypt having stopped supporting it in 2025.
P
Page builder
CMS extension that allows visual drag-and-drop page building without writing code: Elementor, Divi, or WPBakery on the WordPress side. Highly appreciated for their simplicity, page builders often generate heavy code in return (large DOM, superfluous CSS and JavaScript) which penalizes web performance if not optimized.
PageSpeed Insights
Performance measurement tool developed by Google's teams and focused on Core Web Vitals. You can run synthetic tests and access field data (CrUX) for any page, on Desktop and mobile.
PHP
Acronym for "Hypertext Preprocessor," it refers to a programming language used to run a majority of websites. Free and Open-Source, it is at the heart of most current CMS: WordPress, PrestaShop, Magento, Drupal... It is regularly updated to improve performance.
Polyfill
JavaScript tool that allows emulating recent browser features on older browsers that do not natively support them. This family of tools has a significant negative impact on page performance.
Proxy
Tool that interfaces between a web browser and a web server to add new functionalities. Most proxies improve performance and/or security. CDNs like Cloudflare are technically proxies.
PWA
Acronym for "Progressive Web App," it refers to a website that relies on a service worker to offer features similar to a native application: installation on the home screen, offline functionality, push notifications... The interface and experience thus approach what mobile devices offer natively.
R
301 Redirect
Response from a web server indicating to the browser and search engines that a page has permanently changed its address. Essential during a redesign to preserve SEO, it does, however, have a cost: each redirect adds a network round trip. Redirect chains (A to B, then B to C) are particularly detrimental to TTFB.
Resource Hints
Family of HTML attributes that allow indicating to the browser which resources or origins to prepare in advance: "preconnect" to establish a connection, "preload" to load a critical resource, "prefetch" to anticipate the next navigation… Used wisely, they improve speed metrics like LCP; used excessively, they produce the opposite effect.
Resource
A resource is a file requested by the browser (via what is called an HTTP request) to display a web page. Any JavaScript, CSS, font, or image file is thus a resource from the perspective of a web page.
Third-party resource
Resource requested by a page from a different domain name. This is very often JavaScript and fonts, but also sometimes CSS or images.
Robots.txt
This text file, placed at the root of a site, aims to facilitate crawling by search engines. It guides indexing robots by allowing and blocking certain sections, as well as providing links to XML sitemaps.
RUM
Acronym for "Real User Monitoring," which corresponds in French to what is called "field data." It involves collecting performance metrics from real users to generate reports in dashboards. The best-known RUM data source is CrUX, based on data from Google Chrome browser users. However, there are dozens of paid tools that collect data via JavaScript tags, including historical APMs.
S
Search Console
Free Google tool that allows site owners to track their presence in the search engine: indexed pages, queries generating clicks, errors encountered... Its "Core Web Vitals" report, fueled by CrUX data, indicates whether pages meet the set thresholds for Core Web Vitals.
SEO
Acronym for “Search Engine Optimization,” the term refers to the discipline of improving a site's visibility in search engines. Historically very focused on positions in search results pages, SEO has become more complex over the evolution of Google's algorithm. Synonym: natural referencing.
SERP
Acronym for "Search Engine Results Page," it refers to the results page displayed by a search engine following a query. Ranking well there is the goal of SEO. Web performance contributes to this: with equal relevance and quality, Google favors sites that offer the best browsing experience.
Web server
Computer on which a website's data is hosted, accessible to everyone via an Internet connection. Specific software ensures its operation: Apache, Nginx, or LiteSpeed for serving files, MySQL for managing databases, PHP for executing scripts… The owner can connect to it via FTP or SSH for management. See What is considered good hosting.
Service worker
JavaScript script that installs itself in the browser during a site visit and continues to run in the background. It can intercept requests to serve resources from a local cache, enable offline functionality, or receive push notifications. It is the technical component at the heart of PWAs.
SLA
Acronym for "Service Level Agreement." A contractual commitment by a provider regarding the quality of their service, usually expressed as an availability rate: an SLA of 99.9% allows for less than 9 hours of downtime per year. It is a key criterion when choosing hosting.
SPA
Acronym for "Single Page App," it refers to a type of website built in JavaScript that behaves like an application. Components displayed on the current page are dynamically updated via Ajax calls based on user interactions.
Speculation Rules
Browser API that allows anticipating the loading or even rendering of pages based on user actions. The browser uses its own signals to identify the relevance and opportune moment to initiate the process, generally on link hover. This is a very effective feature for improving the performance of multi-page browsing sessions.
Speed Index
Synthetic performance indicator that measures the time it takes for elements to appear within the viewport. As the information is visual and not technical, analysis tools rely on video recordings. Due to its complexity, ratings vary depending on the tool used.
SSR
Acronym for "Server Side Rendering." This refers to performing pre-rendering of dynamic pages on the server side. The technique is used with JavaScript frameworks like React, Angular, or Vue.js to improve performance, particularly LCP. Components are already present on load, before being made interactive via JavaScript later (this is "hydration").
Stack
A stack refers to a set of complementary technologies that allow a web application to function. We generally distinguish between the front-end stack (frameworks, tools, and languages, etc.) and the back-end stack (web server, SSR tools, etc.).
SVG
Vector image format particularly suited for logos, icons, and pictograms. It displays sharply regardless of screen pixel density.
Synchronous
Synchronous loading of a resource executes as soon as it is called, interrupting the browser's page rendering.
T
TBT
Acronym for "Total Blocking Time," this performance metric measures the sum of Blocking Time periods that occur during the loading of a web page. It is often used in synthetic tests as an approximation of INP, as the latter can only be measured via field data.
Template
A template is a set of HTML files used to manage the layout of a website. Some parts are common to the entire site (header, menus, footer, etc.) while others are specific to each type of page: homepage, content archives, articles, product pages, contact page, etc.
TCP
Acronym for "Transmission Control Protocol," it refers to the data exchange protocol at the core of the Internet's operation. It allows web browsers to download resources from a server when you visit a web page. It relies on IP addresses for this. Note: With HTTP/3, browsers now prefer QUIC, a newer protocol based on UDP that speeds up connection establishment.
TLS
Acronym for “Transport Layer Security,” this protocol is responsible for ensuring secure exchanges between a browser and a server. It secures “https” pages by verifying the authenticity of the digital certificate provided by the server.
TTFB
Acronym for “Time To First Byte,” this performance indicator measures the time it takes for a server to respond to the browser's first request. It depends on the server configuration, the presence or absence of cache, but also on the visitor's connection (bandwidth, latency).
Tofu
In typography, it refers to a character that cannot be displayed because the glyph used is not available in the font file, leading to a rendering like □ or ☒. The term “Tofu” is used because this square resembles a block of tofu (the Asian soy cheese).
U
UI
Acronym for “User Interface.” It refers to all the graphical elements of a web page. The structure, typography, images, and colors are all elements of the UI.
UX
Acronym for “User eXperience.” It refers to how a user interacts with a web page. A quality UX involves relevant choices in terms of ergonomics, accessibility, and adherence to conventions and best practices.
V
VDS
Acronym for “Virtual Dedicated Server,” which translates to “serveur dédié virtuel” in French. It’s a type of hosting between a VPS, which is performant and flexible, and a dedicated server, which is very performant but less flexible. The main difference between a classic VPS and a VDS is that the latter has rigorous isolation, whereas some VPS offerings rely on dynamic partitioning, with resources that can fluctuate depending on the load (CPU, RAM, and IO).
Viewport
The area of the current page visible in a web browser. Upon loading, the viewport corresponds to the area above the fold.
VOC
Acronym for "Voice Of Customer." It is a family of tools aimed at collecting user feedback on a site and anticipating their potential frustrations. They usually manifest as a button displayed in a fixed position, providing access to a chat window. By extension, chatbots and conversational robots are often included.
VPS
Acronym for “Virtual Private Server,” which translates to “serveur privé virtuel” in French. It’s a type of hosting between shared hosting, which is not very performant, and a dedicated server, which is very performant but also very expensive. A VPS offers an interesting compromise between performance, cost, security, and scalability: since it’s not a physical machine, you can perform instant backups or scale the configuration on the fly.
W
W3C
The W3C, or "World Wide Web Consortium," is the international organization responsible for overseeing web technologies. It is behind standards like XML, CSS, and SVG, which it regularly evolves. The HTML standard, meanwhile, has been maintained since 2019 by the WHATWG, with which it collaborates.
WAF
Acronym for "Web Application Firewall." Placed in front of the web server, it analyzes incoming requests to block attacks (injections, malicious bots, DDoS, etc.) before they reach the site. It also protects performance: parasitic traffic consumes server resources at the expense of real visitors.
Waterfall
Translates to "waterfall chart": a graphical representation of the loading of a web page, where each line represents an HTTP request and its precise timeline. It is the reference tool for understanding how the browser loads a page and identifying bottlenecks. It can be found in browser developer tools (Network tab) and in testing tools like WebPageTest.
WCAG
Acronym for "Web Content Accessibility Guidelines", these guidelines aim to define the accessibility criteria of a website for people with disabilities and seniors. They are defined by the W3C.
Webfont
Font downloaded by a web page to display all or part of its texts. The best practice is to offer it in Woff2, the lightest format, supported by all browsers today. We also recommend implementing font-display: swap to allow text to be displayed before the webfont is downloaded. For more information, see our article on optimizing web fonts.
Web performance
Technical discipline aimed at ensuring fast display and good fluidity of web pages in browsers, during their loading and throughout their use. For a complete definition, consult our article "What is web performance?".
Z
Zstandard
Modern compression algorithm used by web servers to reduce the size of text-encoded resources (html, CSS, JavaScript...), and thus their download time by browsers. Its compression ratio is between those of Gzip and Brotli, but it compresses much faster than the latter, consuming less CPU resources. It is thus particularly suitable for dynamic content, such as html pages themselves.
