HTML <dfn> Tag with a nested <abbr> Tag

<p><dfn><abbr title="HyperText Markup Language">HTML</abbr></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 while defining an abbreviation.

As demonstrated above, you can nest the <abbr> element inside the <dfn> element to define an abbreviation. The abbreviation is provided as the <abbr> element's contents and the title attribute provides the expansion of the abbreviation.

About 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.

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