aristocracy vs upper crust

aristocracy and upper crust both are nouns.

Word	Noun	Adjective	Verb	Adverb
aristocracy	Yes	No	No	No
upper crust	Yes	No	No	No

As nouns, upper crust is a hypernym of aristocracy; that is, upper crust is a word with a broader meaning than aristocracy:

aristocracy: the most powerful members of a society
upper crust: the class occupying the highest position in the social hierarchy

Other hypernyms of aristocracy include upper class.

aristocracy (noun)	upper crust (noun)
a privileged class holding hereditary titles	the class occupying the highest position in the social hierarchy
the most powerful members of a society