lower class vs underclass

lower class and underclass both are nouns.

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

As nouns, lower class and underclass are synonyms defined as:

lower class and underclass: the social class lowest in the social hierarchy

lower class (noun)	underclass (noun)
the social class lowest in the social hierarchy	the social class lowest in the social hierarchy

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