any of a genus (Xiphophorus) of fishes that are livebearers: the male has the lower caudal fin rays drawn out in a swordlike process
See swordtail in American Heritage Dictionary 4
noun
A small, brightly colored, live-bearing freshwater fish (Xiphophorus helleri) of Central America, having a long tapering extension of the caudal fin in the male and popular in home aquariums.