Fi·la·ri·a n.; pl. filariae. Zool. a small, slender nematode worm of the family Onchocercidae (Filariidae) of many species, parasitic when adult in various animals, including man. They may live within the blood, or in other bodily fluids, or within tissues or cavities of the body. Infection with such organisms may be transmitted by blood-sucking arthropods.