mother superior vs abbess

mother superior and abbess both are nouns.

Word	Noun	Adjective	Verb	Adverb
mother superior	Yes	No	No	No
abbess	Yes	No	No	No

As nouns, mother superior and abbess are synonyms defined as:

Other synonyms of mother superior include prioress.

mother superior (noun)	abbess (noun)
the superior of a group of nuns	the superior of a group of nuns