A citation is any place another site names you: in an article, a list, a study or a forum answer. Some citations include a link and some don't.
Citations matter even without a link. They help people discover you, they build familiarity with your name, and AI assistants and search features draw on what's written about you across the web when they recommend products.
Unlinked citations are also the easiest links to win: the writer already decided you were worth mentioning, so asking them to add a link is a small request.
Example
A PhD student's blog saying "I keep all my notes in Margin" is a citation, linked or not.
Putting it into practice
Where to use it: Unlinked mention email template and Expert quote email template. For the longer version, read getting your brand named in AI search answers.