<dfn>

<dfn> marks a term at the point where it is defined. The surrounding paragraph, term-description pair, or section supplies the definition; the element wraps the term, not necessarily the whole explanation.

Define and reuse a term

The second paragraph links to the defining occurrence through its ID.

<p>
  A <dfn id="grid-track">grid track</dfn> is the space between two neighboring
  grid lines.
</p>
<p>A column is one kind of <a href="#grid-track">grid track</a>.</p>

dfn {
  font-style: italic;
}

How the term is identified

Ordinarily, the element’s text is the term. If dfn has title, that attribute must contain only the term being defined, not its definition. If dfn contains only an <abbr title="…">, that expansion can identify the term.

It accepts phrasing content but must not contain another dfn. Browsers commonly render it in italics; styling is separate from its definition meaning.

Avoid hidden definitions

A tooltip alone is not a usable definition for every reader. Explain the term visibly and use abbr for abbreviation expansion where appropriate.

Try adding another sentence to the definition paragraph. The definition can be longer while dfn still marks just the term. Use dl for a glossary of several term-description groups.

Sources and browser support

MDN: dfn describes the syntax and links to the specification and compatibility data. Check individual values in that table when supporting older browsers.