Gnat Gnat, n. [AS. gn[ae]t.] 1. (Zo["o]l.) 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. 2. 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. Gnat catcher (Zo["o]l.), one of several species of small American singing birds, of the genus Polioptila, allied to the kinglets. Gnat flower, the bee flower. Gnat hawk (Zo["o]l.), the European goatsucker; -- called also gnat owl. Gnat snapper (Zo["o]l.), a bird that catches gnats. Gnat strainer, a person ostentatiously punctilious about trifles. Cf. --Matt. xxiii. 24.