any of a family (Poeciliidae, order Atheriniformes) of small, tropical, American, freshwater bony fishes that bear live young rather than lay eggs, as the guppy, swordtail, and molly: commonly kept as aquarium fishes
See livebearer in American Heritage Dictionary 4
noun
A fish, especially of the family Poeciliidae, that bears live young rather than depositing eggs.