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One of the control surfaces on an aircraft, used primarily to control roll. There is one on the trailing edge of each wing and they are attached to the control column such that moving the latter left-to-right moves one aileron up and the other down.

Moving an aileron down increases the lift on that wing, along with the induced drag, thus causing the ailerons to have yaw as a secondary effect. Likewise moving an aileron up (on the opposite wing) reduces the lift on that opposite wing.

The lift differential between the two wings causes a torque upon the fuselage, rolling it around the front-to-rear axis.

Other major control surfaces (excluding powered stuff like propeller pitch, throttle) are

Secondary effects are most noticeable on aircraft with large wingspan, eg gliders and are a cause of pot stirring. co-ordinated control is what you need to overcome secondary effects and make it look like you're actually in control of the aircraft.