thicket vs underbrush

thicket and underbrush both are nouns.

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

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

thicket: 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 thicket include brake, canebrake, spinney, undergrowth, underwood.

thicket (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