Plain English Meaning

A small flap on the back edge of a control surface that the pilot adjusts so the aircraft will hold a steady attitude on its own, without the pilot having to keep pushing or pulling on the controls.

Full Definition

A small, adjustable surface attached to the trailing edge of a primary flight control surface (such as an elevator, rudder, or aileron) that the pilot moves to relieve the control pressure needed to hold the aircraft in a desired attitude. When the trim tab is deflected in one direction, aerodynamic force on the tab pushes the larger control surface in the opposite direction, it in place without continuous input on the controls.