copse vs underbrush

copse and underbrush both are nouns.

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

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

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

Other hyponyms of copse include brake, canebrake, spinney, undergrowth, underwood.

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