highland vs plateau

highland and plateau both are nouns.

Word	Noun	Adjective	Verb	Adverb
highland	Yes	Yes	No	No
plateau	Yes	No	No	No

As nouns, plateau is a hyponym of highland; that is, plateau is a word with a more specific, narrower meaning than highland:

Other hyponyms of highland include down, tableland.

highland (noun)	plateau (noun)
elevated (e.g., mountainous) land	a relatively flat highland

highland (adjective)	plateau (adjective)
used of high or hilly country