lower class vs stratum

lower class and stratum both are nouns.

Word	Noun	Adjective	Verb	Adverb
lower class	Yes	No	No	No
stratum	Yes	No	No	No

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

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

lower class (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)