Some words make sense only if you already know the subject, culture, or context behind them. Technical jargon, idioms, acronyms, and words used in a restricted or non-literal way can prevent users from understanding content, even when sentences are short and grammatically simple.
For many users, especially those with cognitive, language, or learning disabilities, context alone is not enough to infer meaning. When content relies on unusual words without explanation, understanding breaks down.
What WCAG Means by “Unusual Words”
In this criterion, unusual words include:
- Technical or domain-specific jargon
- Idioms and figurative expressions
- Acronyms or abbreviations with non-obvious meanings
- Common words used in a specialised or restricted way
A word does not need to be rare to be unusual. If its meaning cannot be reliably understood by a general audience without prior knowledge, it qualifies.
Examples:
- “Cascade” is used to describe a dictionary lookup order
- “Text” is used as a defined technical term in WCAG
- Idioms like “hit the ground running” or “raise the bar”
WCAG Success Criterion
3.1.3 Unusual Words (Level AAA)
A mechanism is available for identifying specific definitions of words or phrases used in an unusual or restricted way, including idioms and jargon.
This is a Level AAA success criterion. It is not required for Level A or AA conformance, but it significantly improves comprehension.
What WCAG Means by a “Mechanism”
In WCAG, a mechanism is any consistent, reliable way for users to access information.
It does not have to be a specific HTML feature. A mechanism can be:
- An inline explanation
- A linked definition
- A glossary on the same page or a clearly linked page
- An accessible tooltip or disclosure
What matters is that the mechanism:
- Is available when the word appears
- Works with keyboard and assistive technologies
- Explains the meaning as used in that context
What This Criterion Requires
If your content uses words or phrases that readers are unlikely to understand without specialised knowledge, you must provide a way for users to learn what those words mean.
WCAG does not require a specific technique. It requires that users are not forced to guess.
Realistic Example: Failure vs Success
Failure
The system uses a cascade to resolve dictionary lookups.
A non-expert reader has no way to know what “cascade” means here.
Success (Inline Definition)
The system uses a cascade, meaning dictionaries are searched in a predefined order.
Success (Linked Definition)
The system uses a cascade.
(The word “cascade” links to a definition at the bottom of the page.)
Common Ways to Provide Definitions
Inline Definitions
Explain the term directly in the text. WCAG uses the term text in a specific technical sense.
Glossaries
Provide a glossary on the same page or link to it clearly.
WCAG uses the word text in a specific way.
Text: Characters that can be programmatically determined and rendered visually.
Linked Definitions
Link the term to its definition within the page or a glossary.
Tooltips and Disclosures
Tooltips are acceptable only if they are accessible.
To be valid, tooltips must:
- Be reachable via keyboard focus
- Remain visible while focused
- Expose their content to assistive technologies
Hover-only tooltips fail this criterion.
What Does NOT Count as a Valid Mechanism
- A glossary that exists but is not linked from the content
- Definitions available only on another page with no navigation path
- Mouse-hover tooltips with no keyboard access
- Definitions that explain the dictionary meaning, but not the specialised meaning
- Explanations hidden behind gestures or icons with no label
If users must guess or search externally, the requirement is not met.
Why This Matters
Cognitive and Language Disabilities
Some users:
- Have difficulty decoding unfamiliar words
- Cannot infer meaning from context
- Struggle with figurative or abstract language
Unexplained terminology can make otherwise simple content unusable.
Visual Disabilities
Users of screen magnifiers often see only a small portion of the page at a time. This removes the surrounding context that might otherwise help explain a term.
Explicit definitions prevent loss of meaning when context is limited.
Relationship to Other Criteria
- 3.1.1 Language of Page — identifies the language, not word meaning
- 3.1.2 Language of Parts — handles language changes, not terminology
- 3.1.5 Reading Level (AAA) — addresses overall complexity, not specific terms
Unusual Words focuses specifically on word meaning, not language or reading difficulty.
How to Test
- Scan the content for jargon, idioms, and specialised terms
- Ask: Could a non-expert understand this word without prior knowledge?
- Verify that each unusual term has a clear definition mechanism
- Confirm the mechanism works with keyboard and assistive technologies
- Ensure the definition matches how the word is used in context
If understanding depends on insider knowledge, the criterion is not met.
Rule Description
Words or phrases used in an unusual, technical, idiomatic, or restricted way must have a mechanism that allows users to identify their specific meaning as used in the content.
Wrapping Up
Understanding content should not depend on insider knowledge. When unusual words are left unexplained, users are forced to infer meaning, something many cannot reliably do. Providing clear, accessible definitions removes ambiguity and prevents misinterpretation, especially in instructional, technical, legal, and educational content. While WCAG 3.1.3 is Level AAA, applying it consistently makes content clearer, more precise, and easier to understand for everyone.
