HTML <dfn> Tag

<p><dfn>HyperText Markup Language</dfn> is the standard language for marking up web pages and other HTML documents.</p>
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The above example demonstrates usage of the <dfn> element.

The <dfn> element represents a defining instance of a term.

Because it represents the defining instance, the <dfn> element only needs to be applied once (at the time the term is being defined), and not to subsequent occurrences of that term. The above example demonstrates this, with the first occurrence being the defining instance.

You should nest an <abbr> element inside the <dfn> when defining an abbreviation.

Note that, if a <dfn> tag has a title attribute, then the exact value of that attribute is the term being defined.