noun 1. any of a phylum (Nematomorpha) of elongated worms that have separate sexes, are parasitic in arthropods as larvae, and are free-living in water as adults — called also horsehair worm 2. any of a genus (Capillaria) of nematode worms that include serious parasites of the alimentary tract of fowls and tissue and organ parasites of mammals

Merriam-Webster's Collegiate Dictionary