dfn

The dfn element indicates the defining instance of a term, marking where a term is first introduced and defined in the document.

Description

The <dfn> element identifies the defining instance of a term. The paragraph, definition list group, or section containing the <dfn> should also contain the definition. Styled with italic and semi-bold (font-weight: 500) to distinguish it from regular italic text like  or .

When to Use

First use of technical terms: Define jargon when introduced
Glossary entries: Mark terms in a glossary (inside <dt>)
Educational content: Introduce new vocabulary or concepts
Documentation: Define product-specific terminology

When NOT to Use

For emphasis — use  or 
For repeated uses of the term — only mark the defining instance
For abbreviations alone — use <abbr> (but wrap <abbr> inside <dfn> when first defining it)

Examples

Title Attribute

When the content of <dfn> differs from the term being defined (e.g., an abbreviation), use title to specify the full term:

CSS Reference

The semi-bold weight (500) distinguishes <dfn> from  (which is just italic) and  (also just italic).

Accessibility

Some screen readers announce defined terms, helping users identify definitions
The surrounding paragraph should contain the actual definition
Adding an id enables linking from other occurrences back to the definition
Search engines can use <dfn> to extract term-definition pairs

<abbr> — For abbreviations (often used inside <dfn>)
<dl> — Definition lists for glossaries
<dt> — Definition term (wraps <dfn> in glossary patterns)
<a> — For linking back to definitions via id
 — For emphasis (not definitions)

dfn