underwood vs thicket

underwood and thicket both are nouns.

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

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

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

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