tribadism
Tribadism is female-to-female genital sex, where the partners' vulvas are maintained in contact with and motion against each other, especially for mutual clitoral stimulation.
Not to be mistagged as frottage, which involves the rubbing of said genitals against any other part of the body; crotch rub, which involves rubbing against foreign objects; or grinding which is rubbing against a penis.
Also known to some as the "scissors position" or "scissoring" since the women's legs look like two pairs of open scissors in the most recognized position.