gnat

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various tiny two-winged flies, especially those that sometimes form swarms.

from The Century Dictionary.

• noun A bird: same as knot.
• noun A small two-winged fly, Culex pipiens, of the family Culicidæ, suborder Nemocera, and order Diptera, called in America mosquito.
• noun Any other insect of the family Culicidæ.—
• noun A nemocerous dipterous insect; a midge.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) 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.
• noun 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.
• noun (Zoöl.) one of several species of small American singing birds, of the genus Polioptila, allied to the kinglets.
• noun the bee flower.
• noun (Zoöl.) the European goatsucker; -- called also gnat owl.
• noun (Zoöl.) a bird that catches gnats.
• noun a person ostentatiously punctilious about trifles. Cf. Matt. xxiii. 24.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any small insect of the order Diptera, specifically within the suborder Nematocera.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (British usage) mosquito
• noun any of various small biting flies: midges; biting midges; black flies; sand flies