underclass vs stratum

underclass and stratum both are nouns.

Word	Noun	Adjective	Verb	Adverb
underclass	Yes	Yes	No	No
stratum	Yes	No	No	No

As nouns, stratum is a hypernym of underclass; that is, stratum is a word with a broader meaning than underclass:

Other hypernyms of underclass include class, social class, socio-economic class.

underclass (noun)	stratum (noun)
the social class lowest in the social hierarchy	an abstract place usually conceived as having depth
	a subpopulation divided into a stratified sampling
	people having the same social, economic, or educational status
	one of several parallel layers of material arranged one on top of another (such as a layer of tissue or cells in an organism or a layer of sedimentary rock)

underclass (adjective)	stratum (adjective)
belonging to the lowest and least privileged social stratum