Plain English Meaning
The flaps near each wingtip that the pilot uses to tilt the airplane left or right. Pushing the control wheel or stick to the left makes the left wing drop; pushing right makes the right wing drop.
Full Definition
A movable control surface hinged to the trailing edge (rear edge) of each wing, near the wingtip, used to roll the airplane around its longitudinal axis. The ailerons move in opposite directions: when one goes up, the other goes down. This produces unequal lift between the two wings, which banks the airplane left or right.
Context Anchor
Seen during the visual preflight assessment when checking that each aileron is attached, moves freely, and responds correctly to the control wheel or stick.