The definition of a gnat is a small flying, biting insect with two wings.
An example of a gnat is a No-see-um.
gnat definition by Webster's New World
any of various small insects, esp. certain dipterous flies, which often bite
Brit. a mosquito
Origin: Middle English ; from Old English gnæt, akin to German dialect, dialectal gnatze, Low German gnatte ; from Indo-European an unverified form ghnedh- ; from base an unverified form ghen- from source gnaw