<dfn>
Define a term, the first time a concept appears
<dfn> — The definition
The <dfn> element identifies the term being defined within the surrounding paragraph. You only use it at the first occurrence, where the definition appears.
When to use it
You are introducing a technical term, concept, or jargon for the first time and providing its definition in the surrounding sentence or paragraph. Subsequent occurrences do not need <dfn>.
What you gain
Accessibility: screen readers do not treat <dfn> differently. The value is in the code: a parsing tool knows that this word is being defined here.
Code example
<p>
A <dfn>semantic element</dfn> is an HTML tag that conveys
meaning about the content it wraps, beyond its visual
presentation.
</p>