The category I'm thinking of is a subset of self-referential; a concept which is encapsulated in its expression, where it is an example of itself: "This is a sentence" or "is sentence fragment" or `<html>`. It's an admittedly fuzzy/arbitrary category that I made up. :)
- "word"
- every Audible book begins with: "this is audible"
- "shibboleth" works perfectly well as a shibboleth