Definition of "swordtail"

swordtail

noun
plural swordtails
1. One of many species of freshwater fish, in genus Xiphophorus, others of which are called platyfish.
  Quotations
  But ongoing efforts to sequence genes from other fishes, including salmon and swordtail, will also help to test the theory, Postlethwait says.
  1998 August 21, Gretchen Vogel, “EVOLUTIONARY BIOLOGY:Doubled Genes May Explain Fish Diversity”, in Science‎, volume 281, number 5380, pages 1119–1121
2. Any of various papilionid butterflies that have a long sword-like projection from the tornal section of each hindwing.