topminnow n 1: small usually brightly-colored viviparous surface-feeding fishes of fresh or brackish warm waters; often used in mosquito control [syn: {poeciliid fish}, {poeciliid}, {live-bearer}] 2: freshwater fish of Central America having a long swordlike tail; popular aquarium fish [syn: {swordtail}, {helleri}, {Xyphophorus helleri}]