underwood vs brushwood

underwood and brushwood both are nouns.

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

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

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

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