underwood vs coppice

underwood and coppice both are nouns.

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

As nouns, coppice is a hypernym of underwood; that is, coppice 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
coppice: a dense growth of bushes

Other hypernyms of underwood include brush, brushwood, copse, thicket.

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