black wid·ow • n. a venomous American spider (Latrodectus mactans, family Theridiidae), the female of which has a black body with a red hourglass shape on its underside.

black widow Common name for a small spider found in many warm regions of the world. It is black and has red hour-glass-shaped marks on the underside. Its bite is poisonous, though rarely fatal to humans. Length: 25mm (1in); the male is smaller. Family Therididae; genus Latrodectus.