They are called autological words.

Autological word:

An autological word (also called homological word)1 is a word that expresses a property that it also possesses (e.g. the word "short" is short, "noun" is a noun, "English" is English, "pentasyllabic" has five syllables, "word" is a word). The opposite is a heterological word, one that does not apply to itself (e.g. "long" is not long, "monosyllabic" has five syllables).

Unlike more general concepts of autology and self-reference, this particular distinction and opposition of "autological" and "heterological words" is uncommon in linguistics for describing linguistic phenomena or classes of words, but is current in logic and philosophy where it was introduced by Kurt Grelling and Leonard Nelson for describing a semantic paradox, later known as Grelling's paradox or the Grelling–Nelson paradox.2

One source of autological words is ostensive definition: the reference to a class of words by an example of the member of the class, as it were by synecdoche: such as mondegreen, oxymoron, eggcorn, bahuvrihi, etc. A word's status as autological may change over time. For example, neologism was once an autological word but no longer is; similarly, protologism (a word invented recently by literary theorist Mikhail Epstein) may or may not lose its autological status depending on whether or not it gains wider usage — Wikipedia

---

Autological words:

Most words have a rather abstract connection to the things they describe. The word “yellow” is not actually yellow. The word “square” is not a square. Nor do we expect them to be. That’s the way of the world and the arbitrary nature of language. But some words do embody the properties they denote. We call them autological words, and they are a self-centered, self-referential bunch. Here are 17 of them.

1. Word: Yup, that’s what it is.

2. English: Well, it ain’t French.

3. Erudite: It is a very scholarly word, this word that means scholarly.

4. Noun: Verb, adjective, and adverb are nouns too.

5. Buzzword: Been hearing this everywhere.

6. Cutesy: Barfsy.

7. Polysyllabic: Lots of syllables in this one.

8. Sesquipedalian: A term for “long word” from the Latin for “a foot and a half long.”

9. Unhyphenated: And should remain so.

10. Magniloquent: Grandiose and pompous indeed.

— Mental Floss