Html dfn tag

The dfn element is used to identify the defining instance of a term—that is, the point in the document at which a given term (most probably an industry-specific word, or jargon of some kind) is first explained. Some, but not all, browsers will render the content inside the dfn element in italics, which is a long-standing typographic convention for presenting the defining instance of a word, particularly in scientific papers. You could use CSS to format the definition content in browsers that don’t style the definition in italics.

Note that dfn is short for defining instance, not for definition—you should not place the actual definition of the term between the opening <dfn> and closing </dfn> tags.

Example

The term “progressive enhancement” is defined in the code below:

<p>The concept of <dfn>progressive enhancement</dfn> has been
    about for a few years. You could say that it describes an
    approach to web development from the point of view that 'the
    glass is half full' rather than 'the glass is half empty.'
    Progressive enhancement came to the public's attention when ⋮</p>