A blood-sucking dipterous fly, of the genus Culex, undergoing a metamorphosis in water. The females have a proboscis armed with needlelike organs for penetrating the skin of animals. These are wanting in the males. In America they are generally called mosquitoes. See Mosquito.

Any fly resembling a Culex in form or habits; esp., in America, a small biting fly of the genus Simulium and allies, as the buffalo gnat, the black fly, etc.

any of various small biting flies: midges; biting midges; black flies; sand flies

another one of those tiny flying insects that hang around your head and tries to sting you, usually succeeding in annoying quantities.

The term gnat is applied as a colloquial name to any of various small insects in the order Diptera and specifically within the suborder Nematocera. This suborder represents the more primitive members of the Dipteran order but still contains several very notable and important families such as the Chironomidae (non-biting midges) and the Culicidae (mosquitoes). Other families include the Tipulidae (crane flies), Bibionidae (hairflies), Ceratopogonidae (biting midges), Cecidomyiidae (gall midges), Simuliidae (black flies), and others.