The dfn element is one of a large group of elements that the HTML 4 recommendation calls phrase elements. Such elements assign structural meaning to a designated portion of the document. A dfn element signifies the first usage of a term in a document (its defining instance). A common technique in documents is to italicize an important vocabulary term the first time it is used in a document. This is generally the place in the document where the term is defined so that it may be used in subsequent sentences with its meaning understood. By default, mainstream browsers italicize all text within a dfn element. You can, of course, easily define your own style for dfn elements with a style sheet rule.

The language being used for the element's attribute values and content. A browser can use this information to assist in proper rendering of content with respect to details such as treatment of ligatures (when supported by a particular font or required by a written language), quotation marks, and hyphenation. Other applications and search engines might use this information to aid the selection of spell-checking dictionaries and the creation of indices.