Level AAA

Jargon, idioms, and restricted-sense terms block comprehension for users with cognitive and learning disabilities, non-native readers, and newcomers to a domain unless a lookup mechanism is attached to the term.

3.1.3 Unusual Words

In Plain Language

3.1.3 Unusual Words (Level AAA) requires a mechanism for identifying the specific definition of any word or phrase used in an unusual or restricted way -- idioms, technical jargon, slang, dialect, or domain-specific usage whose meaning cannot be inferred from surrounding context[1].

"Mechanism" is deliberately broad: a link from the term to a glossary entry, an inline <dfn>, a <dl>/<dt>/<dd> list on the same page, a disclosure via <details>/<summary>, or a link from the term to a dictionary entry all satisfy the criterion. What does not satisfy it is expecting the reader to open a new tab and search.

Why It Matters

  • Users with cognitive and learning disabilities hit jargon, cannot decode it from context, and stop reading. Without a mechanism attached to the term, comprehension breaks at the word, not at the paragraph.
  • Non-native readers of the page's language recognise common vocabulary but not idioms ("in the weeds", "boil the ocean") or field-specific usage ("discovery" in a legal context, "reduction" in a chemistry context). The surrounding sentence rarely disambiguates.
  • Readers new to a domain -- a developer reading procurement copy, a lawyer reading a medical record, a patient reading a discharge summary -- fail on the same terms their peers treat as background knowledge.
  • Screen-magnifier users "lose context when zoomed-in" because the enlarged viewport isolates a single phrase from the surrounding paragraph, so context clues that a sighted reader can scan for disappear[1].
  • Translation engines and screen readers use <dfn> and explicit glossary markup as signals. A term wrapped in <dfn> is a hint to preserve the term verbatim; a plain jargon word is treated as translatable prose and mangled.

Examples

Do: Inline definition with dfn and abbr

A WCAG audit checks your site against internationally recognized accessibility standards.

✔ Abbreviation is expanded on first use -- users can discover the full meaning

<p>A <dfn><abbr title="Web Content Accessibility
  Guidelines">WCAG</abbr></dfn> audit checks your
  site against internationally recognized
  accessibility standards.</p>
Don't: Jargon with no definition mechanism

Ensure your VPAT covers all Level A and AA SCs before the ICT refresh deadline.

✘ VPAT, SCs, and ICT are unexplained -- readers without accessibility domain knowledge cannot understand this

<!-- FAILS: no mechanism to define VPAT, SCs, or ICT -->
<p>Ensure your VPAT covers all Level A and AA SCs
  before the ICT refresh deadline.</p>
Do: Glossary linked from term

The application was rejected due to a laches defense.

Laches
An unreasonable delay in asserting a legal right or claim, which may bar the claimant from relief.

✔ Term links to its definition -- users can find the meaning without prior legal knowledge

<p>The application was rejected due to a
  <a href="#glossary-laches">laches</a> defense.</p>

<!-- Glossary section on the same page or linked page -->
<dl>
  <dt id="glossary-laches">Laches</dt>
  <dd>An unreasonable delay in asserting a legal
    right or claim, which may bar the claimant
    from relief.</dd>
</dl>
Do: Define idioms on first use

The project is "in the weeds" (meaning it is stuck on minor details and not making progress on the larger goal).

✔ Idiom is followed by a parenthetical definition -- no specialized knowledge required

<p>The project is "in the weeds" (meaning it is
  stuck on minor details and not making progress
  on the larger goal).</p>

How to Fix It

  1. Inventory the jargon. Walk the page and flag every term used in a restricted, technical, idiomatic, or domain-specific sense. The test is not "would a specialist know it" but "can a reader outside the field infer the meaning from the surrounding sentence". If not, the term needs a mechanism.
  2. Mark first use with <dfn>. Wrap the defining instance of the term in <dfn>. The element signals "this is where the term is being defined" to assistive technology and to translation tools, and it gives you a stable anchor to link to. Per WCAG 3.1.3 Situation A, defining the first occurrence on the page is sufficient when the term has a single meaning throughout[1].
  3. Maintain a glossary. For content-heavy pages, put definitions in a <dl> with <dt> terms and <dd> definitions, either on the same page or on a dedicated glossary page. Give each <dt> a stable id so individual terms can be linked.
  4. Link the term to its definition. Wrap the unusual word in an anchor pointing at the glossary entry or same-page definition (<a href="#glossary-laches">laches</a>). This is the mechanism the reader invokes -- not a footnote they have to hunt for.
  5. When a term has more than one meaning on the page, define each occurrence. WCAG 3.1.3 Situation B requires a definition mechanism at each use, because the reader cannot tell which sense is meant without one[1].
  6. Do not conflate 3.1.3 with 3.1.4. Abbreviations (WCAG 3.1.4, also Level AAA) are handled by <abbr title="..."> or an expansion on first use. 3.1.3 covers the underlying unusual word or phrase. A term like "WCAG" needs both -- an expansion of the acronym and a definition of the concept.

References

  1. [1] W3C (2023). Understanding Success Criterion 3.1.3: Unusual Words. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/unusual-words.html ↩ ↩ ↩ ↩