The Collaborative International Dictionary of English v.0.48:

Lesbianism \Les"bi*an*ism\, n. (Med.)
   Homosexual relations between women.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

lesbianism
    n 1: female homosexuality [syn: lesbianism, sapphism]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "lesbianism":
   active algolagnia, algolagnia, algolagny, amphierotism,
   autoeroticism, bisexuality, coprophilia, exhibitionism, fetishism,
   heterosexuality, homoeroticism, homosexualism, homosexuality,
   incest, incestuousness, masochism, narcissism, necrophilia,
   paraphilia, passive algolagnia, pedophilia, sadism, sadomasochism,
   sapphism, scotophilia, sexual inversion, sexual normality,
   sexual preference, swinging both ways, transvestitism, tribadism,
   tribady, voyeurism, zooerastia, zoophilia