What HTML element is used to mark up an abbreviation?

The HTML element used to mark up an abbreviation is `<abbr>`. It identifies shortened forms such as “HTML”, “CSS”, or “Dr.” so that the markup communicates their linguistic meaning rather than merely changing their appearance.

The optional `title` attribute can provide the expanded form: `<abbr title="HyperText Markup Language">HTML</abbr>`. Browsers may show that title as a tooltip, but authors should not rely on a tooltip alone because it may be unavailable to keyboard users, touch users, or some assistive technologies. The visible text should remain understandable in context.

A frequent mistake is choosing `<acronym>`, which was used in older HTML but removed from HTML5. `<dfn>` marks the defining instance of a term, while `<cite>` identifies the title of a creative work. `<abbr>` covers abbreviations broadly, including acronyms, initialisms, and shortened words.