underbrush vs copse

underbrush and copse both are nouns.

Word	Noun	Adjective	Verb	Adverb
underbrush	Yes	No	No	No
copse	Yes	No	No	No

As nouns, copse is a hypernym of underbrush; that is, copse is a word with a broader meaning than underbrush:

underbrush: the brush (small trees and bushes and ferns etc.) growing beneath taller trees in a wood or forest
copse: a dense growth of bushes

Other hypernyms of underbrush include brush, brushwood, coppice, thicket.

underbrush (noun)	copse (noun)
the brush (small trees and bushes and ferns etc.) growing beneath taller trees in a wood or forest	a dense growth of bushes