Allegedly a Type of magic practiced by a lesbian mage Philippa Eilhart (Witcher 2 game character) while trying to heal Saskia (one of the key female characters in Witcher 2).
During the healing ritual that involved casting few spells and using a number of magic ingredients Philippa puts a Rose of Remembrance on Saskia's lips before kissing her - thus finalizing the ritual.
Saskia recovers but her mind remains under control of Philippa's will. All because of the kiss....
Right after the kiss one of the dwarfs that attended the ritual yells "My favourite kind of magic, lesbomancy!" implying that magic practiced by Philippa is based on her sexual orientation...
During the game, Geralt witnesses few scenes in which Philippa was practicing Lesbomancy with her assistant Cynthia, one of such scenes involved use of whip...Thus we can assume that besides being a type of magic, lezbomancy is also any sexual activity (that might or might not imply use of magic to enhance the experience) practiced by two lesbian mages...
My favourite kind of magic, lesbomancy!