Unusual Words
Provide a mechanism for identifying definitions of words used in an unusual or restricted way — jargon, idioms, technical terms. A glossary, inline definitions, or linked definitions all satisfy this AAA criterion.
What it asks
When content uses words or phrases in an unusual way — domain jargon, idiomatic expressions, words restricted to a particular field — the page must offer a way for readers to find a definition. The mechanism can be a linked glossary, inline definitions, a <dfn> element with a popover, or any technique that makes the meaning available on demand.
The goal is to support users with cognitive disabilities, dyslexia, or non-native readers who may not infer meaning from context.
How to meet it
- Maintain a site-wide glossary and link the first occurrence of each defined term on each page.
- Use the HTML
<dfn>element to mark the defining instance of a term, paired with a description nearby. - For acronyms and abbreviations, see 3.1.4 — but the underlying mechanism (definition on demand) is the same.
- For idioms (“hit the ground running”), provide a plain-language paraphrase the first time it appears.
- Avoid hover-only tooltips as the sole mechanism — they fail for touch and keyboard users; pair with a click or focus reveal.
Common failures
- Specialist content (medical, legal, financial) full of jargon with no glossary anywhere.
- A glossary that exists but is hidden three clicks deep with no in-text linking.
- Idioms in user-facing UI copy (“we’re swamped today”) with no plain-language equivalent.
- Defining a term only on hover, so keyboard and touchscreen users can’t reach it.
Why it matters
This is a AAA criterion, so it is not part of typical compliance targets — but it is the right thing to do for any content aimed at a general audience. Users with dyslexia, aphasia, traumatic brain injury, ADHD, and non-native readers all benefit. Sites that publish technical or medical content for lay audiences (health portals, government services, educational resources) should prioritize this even though it sits above the AA bar.