highland vs upland

highland and upland both are nouns.

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

As adjectives, highland and upland are synonyms defined as:

As nouns, highland and upland are synonyms defined as:

highland (noun)	upland (noun)
elevated (e.g., mountainous) land	elevated (e.g., mountainous) land

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