A sustainable web design glossary should do one thing well: tell you what a term means in two sentences, then send you to whoever has already written the long version.
That is the job this page does. Across search, AI, carbon and accessibility, the same handful of terms get used loosely, borrowed between disciplines, or presented by marketing glossaries as if they had an official definition when none exists. This sustainable web design glossary names fifteen terms, sources each one where a source exists, and says plainly where it does not.
Each entry links out to the Agnikii post or service page that covers the term in depth. This page defines; it does not compete for the same ranking as the posts it points to.
Search and AI terms
Core Web Vitals
Core Web Vitals is Google’s set of metrics for real-world loading performance, interactivity and visual stability, as set out in Google’s own Web Vitals documentation. The current three are Largest Contentful Paint (should occur within 2.5 seconds), Interaction to Next Paint, or INP (should be 200 milliseconds or less), and Cumulative Layout Shift (should be 0.1 or less). INP replaced First Input Delay as the interactivity metric in March 2024, so any source still naming FID as the third Core Web Vital is out of date.
Read the full guide: Core Web Vitals.
Answer engine optimisation (AEO)
No standards body, vendor or academic source defines answer engine optimisation. It is an industry-coined term, not a technical specification from Google or any other platform. The most defensible working definition: the practice of structuring content so that AI answer engines can cite it directly in a generated response, as distinct from SEO’s goal of a ranked, clicked link.
Read the full guide: Answer Engine Optimisation: A Practical Guide to AEO.
SEO ranking
Google’s own guidance on ranking says its automated ranking systems look at many factors and signals about hundreds of billions of web pages and other content in its Search index to present the most relevant, useful results. There is no standalone, quotable definition of “SEO ranking” as an isolated phrase distinct from the wider ranking process.
Read the full guide: SEO Ranking Factors in 2026.
Structured data (schema markup)
Google’s own documentation on structured data states that structured data is a standardised format for providing information about a page and classifying the page content. Google describes it as a way to provide explicit clues about the meaning of a page. Nothing in that definition is specific to AI answers: it clarifies what a page is about for whichever system reads it.
Read the full guide: Structured Data for SEO.
Crawling (in SEO)
In Google’s guide to how Search works, crawling is the first of three stages: Google downloads text, images and videos from pages it found on the internet with automated programs called crawlers. The program that does the fetching is called Googlebot, and Googlebot uses an algorithmic process to determine which sites to crawl, how often, and how many pages to fetch from each site.
Read the full guide: How Search Engines Work.
Generative engine optimisation (GEO)
Generative engine optimisation has a genuine academic origin, which sets it apart from AEO. The term was introduced in 2023 in a paper by Aggarwal, Murahari, Rajpurohit, Kalyan, Narasimhan and Deshpande. The paper was accepted to KDD in 2024 and introduced Generative Engine Optimization (GEO) as a paradigm to aid content creators in improving their content visibility in generative engine responses. It evaluates GEO against GEO-bench, a benchmark of user queries it introduces itself. This is a single study, not an industry standard, and its findings describe a benchmark result rather than a guarantee for any individual site.
Read the full guide: Generative Engine Optimisation.
Carbon and sustainability terms
Sustainable web design
The closest reference for sustainable web design is the Web Sustainability Guidelines (WSG) from the W3C (World Wide Web Consortium), the body that sets web standards. The current version was published by the Sustainable Web Interest Group as a Group Note Draft, dated 29 July 2026. A Group Note Draft is a working document, not a finished standard. Group Note Drafts are not endorsed by W3C nor its Members, and the document describes itself as a work in progress. Any claim that a site is certified against it overstates what the guidelines are.
Find out how we build to this: Sustainable Web Design.
Green web hosting
According to the Green Web Foundation, it offers a free verification service for hosting providers who wish to become verified green hosting providers and get listed in its Green Web Dataset and Green Web Directory. According to the Green Web Foundation, verified providers give it evidence on a yearly basis, or better. The evidence it accepts can change, so check its current criteria rather than relying on an old listing.
Read the full guide: Green Web Hosting UK.
Digital carbon footprint (website carbon footprint)
A digital carbon footprint is not measured directly; it is modelled. A widely used method, the Sustainable Web Design model (SWDM), estimates emissions from data transfer across data centres, networks and devices. Website Carbon’s published global average is about 0.36g of CO₂ per page view (Website Carbon, 2025).
Read the full guide and see how Agnikii models this for a single site: Website Carbon Footprint.
WCAG 2.1 Level AA
WCAG 2.1 is the Web Content Accessibility Guidelines, published by the W3C as a Recommendation, its term for a finished standard. At the top of the W3C’s Recommendation are four principles that provide the foundation for web accessibility: perceivable, operable, understandable and robust, often shortened to POUR. Three levels of conformance are defined: A (lowest), AA, and AAA (highest). Level AA is the target most organisations work to, and the full guide sets out how many success criteria sit at each level.
Read the full guide: WCAG 2.1 Level AA.
Agnikii’s own terms
These five terms are coined and used by Agnikii. They have no external source because none is claimed; each is defined operationally on the service page that delivers it.
Carbon Delta
Carbon Delta is the difference between the heavy site a client had and the optimised one they end up with, shown as before-and-after data on measured page weight and modelled carbon per page view. It can be exported into an environmental, social and governance (ESG) reporting framework or a formal tender response.
Find out how we deliver it: Website Sustainability Audits.
Green Performance Method
The Green Performance Method is Agnikii’s five-stage project process:
- Audit: measure what is there.
- Debloat: strip what is slowing it.
- Rebuild: rebuild lean.
- Optimise: tune for speed.
- Sustain: hold the line afterwards.
Find out how it works: Sustainable Website Development.
Agnikii DriftGuard
Agnikii DriftGuard is the ongoing maintenance product that stops digital drift after launch, through staged updates, security hardening, verified backups, Core Web Vitals tracking, accessibility checks and monthly carbon reporting.
Find out what it covers: Sustainable Website Maintenance.
Digital Drift
Digital Drift is the gradual decline in speed, security and accessibility that affects every site after launch. It is gradual by design, which is why it is rarely noticed until rankings or conversions have already dropped.
Find out how we stop it: Sustainable Website Maintenance.
Digital Drag
Digital Drag is a distinct term from Digital Drift. It is the accumulated bloat, oversized media, redundant code and heavy third-party scripts that force devices and networks to work harder than needed, slowing load times, damaging rankings and raising emissions at the same time.
Find out how we remove it: Website Sustainability Audits.
What this sustainable web design glossary leaves out
A sixteenth candidate term, “carbon receipts,” is not defined here. The research behind this glossary found it listed as a preferred term in Agnikii’s own terminology guidance, but no node in the brain gives it a definition. Stating a definition anyway would mean inventing one, which this glossary does not do. That is a gap for a future update, not an oversight to paper over.
This sustainable web design glossary will grow as more terms earn a sourced, two-sentence entry. If a term you expected is missing, it is more likely waiting on a source than forgotten.
Our own work follows the Sustainable Web Design approach described above: lean, accessible, and built from measured page weight, with carbon modelled from it.