3.1.3 Unusual Words
Level AAA · added in WCAG 2.0
Give people a way to find the meaning of jargon, idioms and words used in an unusual or restricted way. An inline definition, a glossary link or a linked dictionary entry all work. This is a level AAA criterion.
Who it helps
People with cognitive, language or learning disabilities may read idioms literally or not know specialist terms. Deaf people whose first language is a sign language, and people reading in a second language, benefit too.
How to meet it
- Define jargon the first time you use it, right in the sentence.
- Mark the defining instance of a term with the dfn element.
- Link unusual terms to a glossary entry on your site.
- Replace idioms like "hit the ground running" with plain wording, or explain them.
- If a term means different things in different places on the page, explain each use.
- Level AAA isn't required by most laws or policies, which point to AA. It's still worth meeting, and it makes technical content easier for everyone.
Examples
<p>Every API call is idempotent, so retries are a piece of cake.</p><p>Every API call is <dfn>idempotent</dfn>: sending the same request
twice has the same effect as sending it once. That makes retries safe.</p><p>Every API call is
<a href="/glossary/#idempotent">idempotent</a>, so retries are safe.</p>Can a scanner detect this?
No
axe-core has no rule for this criterion, because only a person can judge which words are unusual for your audience. Reading tools can flag rare words, but you still need to review the content and decide what to define.
No automated rule tests this criterion. Check it by hand.
Common mistakes
- Assuming your audience knows internal product names or industry abbreviations.
- Writing a glossary but never linking to it from the terms in the text.
- Using a title attribute tooltip as the only definition, which keyboard and touch users often can't reach.
- Forgetting idioms and metaphors, which are hard for people who read language literally.
Related criteria
Check your own page
See which automated WCAG checks your page fails, with the failing elements and how to fix them.
Reviewed November 2, 2026