black widow

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of several poisonous spiders of the genus Latrodectus, the female of which produces toxic venom and has a black shiny body with red markings, often in the shape of an hourglass on the abdomen.
• noun A woman who kills her husband or lover, especially one who kills multiple husbands or lovers.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of several species of venomous spider, particularly Latrodectus sp.. The bite is painful to humans, rarely fatal to healthy individuals, but can be life-threatening or fatal to infants or young toddlers. Females are marked by a red hourglass shape on the abdomen (see photo, this page, and photos in Wikipedia). Mature males are smaller, of the same black color or a dark brown, have an elongated and slender abdomen (rather than the rotund abdomen of the mature female), and are marked on the dorsal of the abdomen with small flattened diamond shapes of the same scarlet color as the female's marks on the ventral of her abdomen.
• noun A woman who kills one or more of her lovers.
• noun A female martyr (a suicide bomber or shaheed), whose husband was killed.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun venomous New World spider; the female is black with an hourglass-shaped red mark on the underside of the abdomen