lesbianism vs homosexuality

lesbianism and homosexuality both are nouns.

Word	Noun	Adjective	Verb	Adverb
lesbianism	Yes	No	No	No
homosexuality	Yes	No	No	No

As nouns, homosexuality is a hypernym of lesbianism; that is, homosexuality is a word with a broader meaning than lesbianism:

lesbianism: female homosexuality
homosexuality: a sexual attraction to (or sexual relations with) persons of the same sex

Other hypernyms of lesbianism include gayness, homoeroticism, homosexualism, queerness.

lesbianism (noun)	homosexuality (noun)
female homosexuality	a sexual attraction to (or sexual relations with) persons of the same sex