swordtail n freshwater fish of Central America having a long swordlike tail; popular aquarium fish Usage(s) About five years ago, when the writer was 17, he did this twice, once on a guppy, once on a swordtail. About five years ago, when the writer was 17, he did this twice, once on a guppy, once on a swordtail. Synonym(s) helleri topminnow Xyphophorus helleri