The HTML element used to define a term in a description list is `<dt>`.
A description list uses `<dl>` as its container, with each term marked by `<dt>` and its corresponding explanation or value marked by `<dd>`. The browser uses this structure to understand the relationship between labels and descriptions, even when CSS changes the visual design.
A common mix-up is choosing `<dl>`, which defines the entire description list, or `<dd>`, which defines the description itself. The element `<li>` belongs to ordinary ordered and unordered lists, such as `<ol>` and `<ul>`, not description lists.
Description lists are useful for glossaries, metadata, product specifications, and question-and-answer pairs. They are semantic HTML: the markup communicates meaning and structure, while CSS controls presentation. HTML's broader element model dates back to Tim Berners-Lee's early HTML work and is now maintained as the WHATWG HTML Living Standard.