What is the definition or description of: black widow spider?

Latrodectus: http://en.wikipedia.org/wiki/latrodectus not all adult black widows exhibit the red hourglass on the ventrum underside or top of the abdomen — some may have a pair of red spots or have no marking at all. The black widow spider is perhaps the best-known member of the genus. Its bite is dangerous because of the neurotoxin latrotoxin, which causes the condition latrodectism, both named for the genus.