1. (Zoöl.) (a) Any winged insect; esp., one with transparent wings; as, the Spanish fly; firefly; gall fly; dragon fly. (b) Any dipterous insect; as, the house fly; flesh fly; black fly. See Diptera, and Illust. in Append.
2. A hook dressed in imitation of a fly, — used for fishing. "The fur-wrought fly." Gay.
3. A familiar spirit; a witch's attendant. [Obs.] A trifling fly, none of your great familiars. B. Jonson.
4. A parasite. [Obs.] Massinger.
5. A kind of light carriage for rapid transit, plying for hire and usually drawn by one horse. [Eng.]
6. The length of an extended flag from its staff; sometimes, the length from the "union" to the extreme end.
7. The part of a vane pointing the direction from which the wind blows.
8. (Naut.)
Defn: That part of a compass on which the points are marked; the compass card. Totten.
9. (Mech.) (a) Two or more vanes set on a revolving axis, to act as a fanner, or to equalize or impede the motion of machinery by the resistance of the air, as in the striking part of a clock. (b) A heavy wheel, or cross arms with weights at the ends on a revolving axis, to regulate or equalize the motion of machinery by means of its inertia, where the power communicated, or the resistance to be overcome, is variable, as in the steam engine or the coining press. See Fly wheel (below).