<DFN> | NN n/a IE 3 HTML 3.2
<DFN>...</DFN> | End Tag: Required
The DFN element is one of a large group of elements that the HTML 4.0 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, Internet Explorer italicizes all text within a DFN element. You can, of course, easily define your own style for DFN elements with a style sheet rule.
Example
    <P>Concerto composers usually provide a space for soloists to show off
    technical skills while reminding the audience of various themes used
    throughout the movement. This part of the concerto is called the <DFN>
    cadenza</DFN>. </P>
Object Model Reference
IE | [window.]document.all.elementID