<article>
A self-contained composition that would make sense on its own: a post, product card, comment, widget.
Baseline: Widely available · Sectioning & landmarks
When to use it
When the chunk would stand alone in an RSS feed or search result; each gets its own heading, and articles nest (comments inside a post).
When not to
Not for mere page chapters — that's section; not for grouping without standalone meaning — that's div.
Example
Overnight Rye Bread
Four ingredients, twelve hours of patience…
<article>
<header><h2>Overnight Rye Bread</h2></header>
<p>Four ingredients, twelve hours of patience…</p>
<footer><address>kitchen@html-tags.com</address></footer>
</article>