The term that is being defined can be any one of the following:
Note that many HTML pages and even a significant number of examples on the web misuse the title attribute for the definition of the term when it is supposed to be the term that is being defined.
title attribute
The title attribute identifies the term that is being defined. Do not use the title tag for the definition of the term or for the expanded meaning of an acronym or abbreviation when it is simply being expanded in the text that follows.
Unlike the common attribute of the same name, the title attribute of the <dfn> tag does not inherit its value from an ancestor tag. However, the title attribute of a child <abbr> tag will be used, which is appropriate when the term being defined is the expanded meaning of an acronym or abbreviation rather than the abbreviation itself.
common attributes
In addition to the attributes above, which are specific to the <dfn> tag, any of the common HTML attributes can also be used.
An example where the term being defined is the content of the <dfn> tag:
<dfn>Love</dfn> is a strong passionate feeling for another person.
An example where the term being defined is specified in the title attribute:
A <dfn id="blog-dfn" title="blog">blog or weblog</dfn> is an online journal.
Whenever a term is used it can refer back to the definition using an <a> tag:
Entries in a <a href="#blog-dfn">blog</a>usually appear in reverse chronological order.
The value of the title attribute should be the term that is being defined. When there is an acronym or abbreviation involved, whether the acronym or its expansion appears in the title tag depends on whether it's the abbreviated form or its expanded meaning that is being defined.
When an abbreviation is a short form of the actual term that is being defined, the title attribute should not be the acronym or abbreviation that stands for the subject of the definition but rather its fully expanded meaning. Since this is the same purpose that the title attribute of the <abbr> tag serves, the title can simply be coded there:
<dfn><abbr title="Internet Corporation for Assigned Names and Numbers">ICANN</abbr></dfn>is the international organization which helps ensure that Internet domain names are assigned in an orderly manner.
When an acronym's meaning is being clarified by expanding it, then it is the acronym itself that is being defined and the title attribute of the parent <dfn> tag also needs to be coded to avoid having it assume the incorrect title value from the <abbr> tag:
When we use the acronym <dfn title="RSS"><abbr title="Really Simple Syndication">RSS</abbr></dfn>it is the abbreviation for <b>Really Simple Syndication</b>, not RDF Site Summary.