Tribadism (Greek: τριβειν, to rub, + ισμος, condition, ism) is a sexual practice where two women rub their external genitalia against one another for clitoral stimulation. Colloquially it is known as scissoring, or less commonly donut bumping, slapping clams or train spotting. In ancient Chinese texts it is often called polishing mirrors. An alternative name for this practice, including other sexual rubbing, is frottage.
The word is derived from the obsolete word tribade, meaning "lesbian", and until the middle of the 20th century (sometime before 1965) meant "lesbianism".