tribadism (n.)

"lesbian sexual activity," 1811, with -ism + tribade (n.), c. 1600, "a lesbian," from French tribade (16c.) or directly from Latin tribas, from Greek tribas, from tribein "to rub, rub down, wear away," from PIE root *tere- (1) "to rub, turn." In reference to a specific sexual technique, from 1965.