undergrowth vs thicket

undergrowth and thicket both are nouns.

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

As nouns, thicket is a hypernym of undergrowth; that is, thicket is a word with a broader meaning than undergrowth:

undergrowth: 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 undergrowth include brush, brushwood, coppice, copse.

undergrowth (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