Glossary
Browse our comprehensive glossary of image optimization, SEO, and web performance terms.
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Accessibility
AI alt text
AI alt text refers to automatically generated alternative text for images using computer vision, optical character recog...
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SEO
AI filename generator
An AI filename generator is a system that uses machine learning and rule-based logic to create descriptive, web‑safe ima...
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Performance
AI image optimisation
AI image optimisation is the use of machine learning models to analyse, enhance, and encode images so they deliver the b...
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Performance
AI upscaling
AI upscaling is the use of machine learning models to increase an image’s resolution while attempting to reconstruct pla...
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Accessibility
Alt text
Alt text is the textual alternative provided via the alt attribute on img and input type=“image” elements. It communicat...
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SEO
Anchor image
An anchor image is a hyperlink whose clickable content is an image. Implemented as an a element containing an img elemen...
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Accessibility
ARIA label
An ARIA label is the accessible name supplied to an element via WAI-ARIA attributes, most commonly aria-label or aria-la...
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Performance
Aspect ratio
Aspect ratio is the proportional relationship between an asset’s width and height, expressed as W:H (e.g. 16:9) or a dec...
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Performance
Aspect ratio property
In CSS, aspect-ratio defines the preferred width-to-height proportion of a box, interpreted as width divided by height. ...
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Compression
Auto-enhance
Auto-enhance is a catch‑all for automated image adjustments that aim to improve perceived quality and consistency withou...
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Formats
AVIF
AVIF (AV1 Image File Format) is a modern still‑image format that stores an AV1 intra frame in a HEIF container. It deliv...
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Performance
Batch processing
Batch processing is the automated handling of large sets of images in grouped jobs rather than on an individual or on‑de...
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Compression
Brotli compression
Brotli is a modern, general‑purpose, lossless compression algorithm and bitstream format standardised by the IETF in RFC...
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Delivery
Cache busting
Cache busting is the practice of changing an asset’s URL—typically by appending a version or content hash—to ensure brow...
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Delivery
Cache-Control headers
Cache-Control headers are HTTP directives that tell browsers, CDNs, and other intermediaries how to cache, reuse, or rev...
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Performance
Caching
Caching is the temporary storage of web resources such as images, CSS, JavaScript, and HTML at or near the client to avo...
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Delivery
CDN (Content Delivery Network)
A Content Delivery Network (CDN) is a distributed platform of edge servers that cache and deliver web assets from locati...
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Formats
Chroma subsampling
Chroma subsampling is a colour-encoding technique that stores chroma (colour difference) at a lower spatial resolution t...
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Performance
CLS (Cumulative Layout Shift)
Cumulative Layout Shift (CLS) is a Core Web Vitals metric that quantifies unexpected visual movement of content while a ...
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Formats
Colour profile (ICC)
An ICC colour profile is a standardised data set that describes how to interpret the colour values in an image by mappin...
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Accessibility
Contrast ratio
Contrast ratio expresses the difference in relative luminance between two colours, typically foreground text and its bac...
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Performance
Core Web Vitals
Core Web Vitals are Google’s standardised user experience metrics for loading, responsiveness, and visual stability. As ...
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Performance
decoding
In web imaging, decoding is the browser process that turns compressed image bytes (JPEG, PNG, WebP, AVIF and others) int...
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Accessibility
Decorative image
A decorative image is a visual element that provides aesthetic value without adding meaning, conveying state, or enablin...
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Infrastructure
Edge Caching
Storing optimised images at global edge servers for speed.
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Formats
EXIF data
EXIF data (Exchangeable Image File Format) is a standardised set of metadata tags embedded in image files that records c...
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Performance
fetchpriority
fetchpriority is an HTML attribute that hints to the browser how urgently a resource should be fetched. It can be applie...
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Formats
HEIC
HEIC is Apple’s common file extension for still images stored in the HEIF container and encoded with the HEVC (H.265) co...
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HTML
Height attribute
In HTML, the height attribute on an img element is a presentational attribute that sets the intended display height in C...
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Performance
HTTP/2
HTTP/2 is a major revision of the HTTP protocol that speeds up the delivery of web assets by changing how requests and r...
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Accessibility
Image accessibility
Image accessibility is the practice of ensuring images convey their purpose and information to people with diverse abili...
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SEO
Image captions
Image captions are visible, human‑readable text presented alongside an image to describe, explain, or add context. They ...
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Delivery
Image CDN
An Image CDN is a content delivery service that fetches original images from an origin, then transforms, compresses and ...
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SEO
Image filename best practices
Image filename best practices are conventions for naming image files so they convey clear, machine-readable meaning whil...
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Performance
Image pipeline
An image pipeline is the end‑to‑end process that prepares, encodes, caches, and delivers images from source assets to us...
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SEO
Image sitemap
An image sitemap is an XML sitemap that extends the standard Sitemap protocol with an image namespace to enumerate image...
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Infrastructure
Image Storage Bucket
Cloud storage for hosting and processing images.
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HTML
Image title tag
Image title tag is a colloquial term for the HTML title attribute applied to an img element. It contains advisory text t...
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Formats
JPEG
JPEG is a family of international standards for compressing continuous‑tone still images, best known on the web for the ...
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Performance
Lazy loading
Lazy loading is a strategy that defers fetching and decoding offscreen images, iframes, or other media until they are ne...
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Performance
LCP (Largest Contentful Paint)
Largest Contentful Paint (LCP) is a Core Web Vitals metric that approximates perceived load speed by measuring the time ...
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Performance
Loading attribute
The loading attribute is an HTML hint on img and iframe elements that influences when the browser fetches a resource rel...
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Compression
Lossless compression
Lossless compression reduces an image’s file size while guaranteeing that decompression reproduces the exact original pi...
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Compression
Lossy compression
Lossy compression reduces file size by irreversibly discarding image information that is less important to human vision,...
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HTML
Manifest.json
Manifest.json, commonly called the Web App Manifest, is a JSON file linked from a site’s HTML that describes how the sit...
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SEO
Open Graph image
An Open Graph image is the preview visual attached to a URL when it is shared on social platforms, chat apps, and other ...
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Performance
Optimised asset
An optimised asset is any web resource—such as an image, font, script, style sheet, or media file—prepared and delivered...
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HTML
Picture element
The <picture> element is an HTML container that enables responsive images through media- and type-based source swi...
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Performance
Pixel density (DPR)
Pixel density, commonly expressed as device pixel ratio (DPR), is the ratio between a device’s physical pixels and CSS p...
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Formats
PNG
PNG (Portable Network Graphics) is a raster image format designed for lossless compression, precise colour, and full alp...
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Performance
Preconnect
Preconnect is a browser resource hint that asks the user agent to open the network path to a target origin before any ac...
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Performance
Preload
Preload is a browser resource hint—declared with <link rel=“preload”> or an HTTP Link header—that instructs the br...
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Formats
Progressive JPEG
Progressive JPEG is a JPEG encoding mode (ISO/IEC 10918-1, progressive DCT; SOF2) that stores an image in multiple scans...
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HTML
Responsive images
Responsive images are a set of HTML patterns and delivery strategies that let the browser choose the most appropriate im...
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Performance
Responsive variants
Responsive variants are multiple encodings of the same image that differ by intrinsic width, pixel density (DPR), file f...
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SEO
SEO image audit
An SEO image audit is a structured evaluation of how images affect a site’s search visibility and performance, with emph...
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HTML
Sizes Attribute
The sizes attribute is an HTML hint used with responsive images to indicate the expected rendered width (the slot size) ...
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Delivery
Smart cropping
Smart cropping is an automated technique for producing image crops that preserve the most important visual content, such...
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HTML
Srcset Attribute
The srcset attribute is a responsive images feature for img and source elements that lists multiple image URLs with desc...
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SEO
Structured data
Structured data is a standardised, machine-readable layer embedded in web pages that describes entities, attributes, and...
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Formats
SVG
SVG (Scalable Vector Graphics) is a text-based, XML vector image format for describing 2D graphics that scale cleanly at...
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Accessibility
WCAG compliance
WCAG compliance, more precisely WCAG conformance, is meeting the testable Success Criteria in the W3C Web Content Access...
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Formats
WebP
WebP is a royalty‑free image format from Google that stores images in a RIFF container (FourCC “WEBP”) using VP8 for los...
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HTML
Width attribute
The width attribute is an HTML attribute that specifies the intended display width of an element, most commonly images, ...