Douglas Harper's Etymology Dictionary
"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, twist" (see throw (v.)). In reference to a specific sexual technique, from 1965.
Wiktionary
n. 1 lesbianism. 2 Specifically, a form of lesbian intercourse involving the rubbing together of two partners' vulvae.
WordNet
n. a form of lesbianism that simulates heterosexual intercourse
Wikipedia
Tribadism or tribbing, commonly known by its scissoring position, is a sex act in which a woman rubs her vulva against her partner's body for sexual stimulation, especially for ample stimulation of the clitoris. This may involve female-to-female genital contact or a female rubbing her vulva against her partner's thigh, stomach, buttocks, arm, or other body part (excluding the mouth). A variety of sex positions are practiced, including the missionary position.
The term tribadism is usually used in the context of lesbian sex, and originally encompassed societal beliefs about women's capability of being penetrative sexual partners. Women accused of having been penetrative during sexual activity were subject to ridicule or punishment. In modern times, the term typically refers to various forms of non-penetrative sex (or frottage) between women without any negative connotations. It may also involve vaginal penetration by use of the fingers, a dildo or double penetration dildo, or refer to a masturbation technique in which a woman rubs her vulva against an inanimate object such as a bolster to achieve orgasm.
Usage examples of "tribadism".
The rest were mostly scenes of tribadism and of lesbian love, and interspersed with them were a few representing flagellation by a girl on a girl, both being stark naked!
There was now no possible doubt that Connie had surrendered herself to the pleasures of tribadism and lesbian love as far as Alice was concerned.