Fly can refer to any of the following things:

• A fly (plural flies) is any species of insect of the order Diptera.
• A fly is a lure, used in angling, with materials such as feathers or cotton thread, tied to look like a real fly.
• Flies in trousers are a zipped, velcroed or buttoned opening; it is used separately, or in conjunction with detaching the button etc. above it, at the waist. It allows dressing and undressing, as well as urinating without undressing. Trousers wide enough to put on and taking them off without having a fly or opening at the side, are either elastic or kept in place with a belt or suspenders. Also some underpants have a fly. These usually do not allow detachment at the waist, elasticity allows putting them on and taking them off. Usually the fly of underpants avoids exposure of the skin just by an overlap of cloth, without buttons, etc. Such a property may be one of the criteria for shorts not to be suitable as outer clothing.
• Flies in a theatre refers to the fly tower over the stage or to a storage are to the side of the stage.
• In baseball, a fly is a ball that is hit into the air. See: fly ball.
• In Flag terminology, fly refers to the end of the flag furthest from the flag pole, or alternately the horizontal length of the flag
• Fly is a model of roller coaster.

• The Fly is the title of two horror movies:
  • The Fly (1958), directed by Kurt Neumann , and
  • The Fly (1986), directed by David Cronenberg, which is a remake of the first one.

In the English language, fly means:

• As a verb, fly means to travel through air in the act of flight, like a bird or aeroplane, or to travel on or pilot an aircraft.
• Fly is also a slang term, meaning "cool" or "hip".