HTML <dfn> Definition Tag

<body>
   ...
   ... phrasing content expected ...<dfn id="dfn-id">... phrasing content ...</dfn>...
   ...
</body>

Make sure you understand the difference between a tag and element and are familiar with the definitions of namespace and other HTML terms.

Code the dfn element where phrasing content is expected.
Begin the dfn element for a definition with a starting <dfn> tag. The element name uses lower case letters and should be in the HTML namespace, which it will pick up automatically from the xmlns attribute on the <html> tag.
Optionally code an id attribute to allow referring back to the defining instance of the term.
Unless the content of the <dfn> tag is a simple text node with the exact term being defined, include a title attribute on the <dfn> tag with the term being defined (see the second example of the <dfn> tag where the term being defined is "blog" but the text content of the <dfn> tag is "blog or weblog").
Include the term being defined as the content within the tags.
End the dfn element with a matching </dfn> closing tag.
Include the definition of the term in the same paragraph or list item outside the <dfn> tag.