Data study · Niche schema types AI answers can read
Only 96 of 772,412 Top-1M Sites, About 1 in 8,000, Mark Up a Glossary With DefinedTermSet
DefinedTerm or DefinedTermSet markup appeared on 324 top-1M sites in October 2026, up from 115 in 2023. Only 96 used a DefinedTermSet, the structure for an actual glossary, and the median page marked up 3 terms.
Analysis of 489 pages with DefinedTerm markup across 324 top-1M sites · HTTP Archive, October 2026, with trend back to 2023
Key findings
- 1
96 of 772,412 top-1M sites (about 1 in 8,000) had DefinedTermSet glossary markup on a crawled page in October 2026.
- 2
Sites with any DefinedTerm or DefinedTermSet markup grew from 115 in 2023 to 324 in 2026 (+181.7%).
- 3
The median page with this markup defines 3 terms; 27.8% define just one and 4.9% define between 21 and 100.
- 4
Only 35.0% of pages give their terms a description, the part that actually defines them.
- 5
Much of the markup is not a glossary at all: DefinedTerm is used as the value of about on 142 pages, knowsAbout on 78 and jobTitle on 46.
- 6
None of the top 1,000 sites had it on a crawled page; 17 sites ranked 1,001 to 10,000 did.
Summary
Picture an insurance broker who writes a plain-English page explaining 40 terms customers trip over: deductible, subrogation, umbrella policy. People find it useful. When someone asks an AI assistant "what is subrogation in car insurance," that page could be a perfect source, if machines can tell it is a set of definitions.
Schema.org has types for exactly this. A DefinedTermSet is a glossary, and each DefinedTerm inside it has a name and a description. It is a clean, structured way to say "this page defines these words."
Almost no one uses it. In October 2026 just 96 of 772,412 top-1M sites had DefinedTermSet markup on a crawled page.
What we measured
We used four October crawls from HTTP Archive (2023 to 2026), mobile client, top-1M sites. HTTP Archive crawls each site's homepage plus one inner page, and we counted both. We parsed the rendered JSON-LD, flattened nested objects and @graph arrays, and flagged pages with DefinedTerm or DefinedTermSet nodes.
For 2026 we counted DefinedTerm nodes per page, recorded the properties each term carries, and noted which property each term is attached to, which shows whether it is part of a glossary or used as a value somewhere else.
Adoption almost tripled, from a tiny base
Most of the growth came in the last year.
Count
| Crawl | Sites crawled | Homepages with markup | Inner pages with markup | Sites with markup |
|---|---|---|---|---|
| Oct 2023 | 900,377 | 69 | 69 | 115 |
| Oct 2024 | 821,066 | 87 | 75 | 117 |
| Oct 2025 | 792,964 | 111 | 115 | 168 |
| Oct 2026 | 772,412 | 229 | 260 | 324 |
Source: aisearchoptimization.net analysis of 489 pages with DefinedTerm markup across 324 top-1M sites · HTTP Archive, October 2026, with trend back to 2023
Even now, sites with any DefinedTerm markup are 0.042% of the top 1M.
Most pages define a handful of terms
True glossaries with dozens of entries are rare.
Pages (N=489)
| Terms | Pages | Share % |
|---|---|---|
| 0 (set only) | 12 | 2.5 |
| 1 | 136 | 27.8 |
| 2 to 5 | 219 | 44.8 |
| 6 to 20 | 97 | 19.8 |
| 21 to 100 | 24 | 4.9 |
| More than 100 | 1 | 0.2 |
Source: aisearchoptimization.net analysis of 489 pages with DefinedTerm markup across 324 top-1M sites · HTTP Archive, October 2026, with trend back to 2023
165 pages on 96 sites wrap their terms in a DefinedTermSet.
Many terms are tags, not definitions
DefinedTerm is often used to label a topic, a skill or a job title rather than to define a word. Counts are pages; a page can use terms in more than one way.
| Measure | Pages | Share % |
|---|---|---|
| Value of about (page topic) | 142 | 29 |
| Value of knowsAbout | 78 | 16 |
| Listed in a DefinedTermSet (hasDefinedTerm) | 53 | 10.8 |
| Value of jobTitle | 46 | 9.4 |
| Value of educationalLevel | 18 | 3.7 |
| Value of mentions | 16 | 3.3 |
| Value of keywords | 13 | 2.7 |
| Term has name | 470 | 96.1 |
| Term has inDefinedTermSet | 226 | 46.2 |
| Term has @id | 225 | 46 |
| Term has description | 171 | 35 |
| Term has url | 112 | 22.9 |
| Term has sameAs | 60 | 12.3 |
| Term has termCode | 45 | 9.2 |
Source: aisearchoptimization.net analysis of 489 pages with DefinedTerm markup across 324 top-1M sites · HTTP Archive, October 2026, with trend back to 2023
A glossary term without a description gives a machine a word and nothing to quote.
What this means
If your site has a glossary or a set of definition pages, mark them up as a DefinedTermSet with one DefinedTerm per entry. Give every term a name, a description that matches the visible definition, and a url pointing to its anchor or page. Keep definitions short and plain so they can be quoted as they are.
"What is" questions are some of the most common prompts people send AI assistants, and a clearly marked glossary is an easy source to use. A free AI visibility audit can show which of your definitions AI engines already pick up, and an AI SEO consultant can mark up the rest.
Limits
HTTP Archive crawls one homepage and one inner page per site, so glossary pages deep in a site are often missed. Counts reflect the sample.
The top-1M list changes between crawls.
We count JSON-LD only; glossaries marked up in microdata or RDFa are not counted.
Site counts for small groups are low, so year-on-year changes can be driven by a few sites.
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Cite this study
APA
Grenyo, K. (2026). Only 1 in 8,000 Top Sites Marks Up a Glossary. AI Search Optimization. https://aisearchoptimization.net/resources/data-studies/glossary-definedterm-markup-adoption
Plain link
Only 1 in 8,000 Top Sites Marks Up a Glossary - AI Search Optimization - https://aisearchoptimization.net/resources/data-studies/glossary-definedterm-markup-adoption
Frequently asked questions
What is DefinedTermSet schema?
A schema.org type for a glossary or controlled vocabulary. Each entry is a DefinedTerm with a name, description and optional code or URL.
How many sites use glossary schema markup?
96 of 772,412 top-1M sites had DefinedTermSet markup on a crawled page in October 2026. 324 had some DefinedTerm or DefinedTermSet markup.
How many terms do glossary pages mark up?
The median page with DefinedTerm markup has 3 terms. 27.8% have one and 4.9% have 21 to 100.
Do glossary terms in markup include definitions?
Often not. Only 35.0% of pages with DefinedTerm markup give any term a description.
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