underwood vs copse

underwood and copse both are nouns.

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

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

underwood: 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 underwood include brush, brushwood, coppice, thicket.

underwood (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