<dfn> elementThe <dfn> element marks the defining instance of a term. It is used at the point where the page is not merely using a word, but explaining what that word means.
This is especially useful in teaching material, reference pages, glossaries, technical explanations, and scientific writing. The element says, "this is the term being defined here."
The nearest enclosing paragraph, description-list group, or section must also contain the definition. If <dfn> appears in a paragraph, that paragraph needs to explain the term. In a glossary, it can appear inside <dt>, with the definition in the same group's <dd>.
The optional title attribute has a special meaning on <dfn>. If it is present, it must contain the exact term being defined, not a general tooltip or extra comment. This is stricter than the casual use of title on many other elements.
If the term is an abbreviation, the elements can work together: <dfn><abbr title="Cascading Style Sheets">CSS</abbr></dfn>. When <dfn> has no title and contains only that <abbr>, its expansion supplies the term being defined. Otherwise, when neither source of a title applies, the text inside <dfn> supplies the term. The surrounding content must still explain its meaning; expanding an abbreviation alone is not a definition.
The <dfn> element should not be used every time an important word appears. It belongs at the defining moment. Later uses can simply use the word, or link to an id on its definition. The example below shows both the definition and a later reference to it.
Attributes: title, GLOBALS.
Typical layout: <p> A <dfn id="selector">selector</dfn> is the part of a CSS rule that chooses the elements to style. </p> <p> A <a href="#selector">selector</a> can match more than one element. </p>
Visual representation:
A selector is the part of a CSS rule that chooses the elements to style.
A selector can match more than one element.