Tag Description
<dfn> Definitions are used to indicate a term that the user of the website may not know. Hovering over the term should bring up a tooltip with a brief description of the term.

The <dfn> tag surrounds the word/term being defined.

Example:

HTML Code Display
In order to gain access to this site, you will need an access code and your <dfn title="A unique number that AFSC uses to identify the business you are operating on behalf of.">AFSC ID</dfn>. In order to gain access to this site, you will need an access code and your AFSC ID.