Definition
Small, adjustable surfaces attached to the trailing edge of a primary flight control (such as the elevator, rudder, or aileron) that the pilot moves to relieve continuous control pressure. By deflecting into the airflow opposite to the desired control position, a trim tab produces an aerodynamic force that holds the main control surface in place, allowing the aircraft to maintain a chosen attitude without the pilot holding pressure on the yoke or pedals.
Plain English
Tiny moveable flaps on the back edge of the main flight controls. The pilot adjusts them so the aircraft will hold a desired pitch, yaw, or roll on its own, without the pilot having to keep pushing or pulling on the controls.
Context Anchor
You encounter trim tabs during preflight inspection, aircraft control checks, and in flight when adjusting trim to reduce control pressure.
Derivation
Trim' comes from the old nautical sense of adjusting a ship so it sits properly in the water -- balanced and steady. 'Tab' simply means a small flap or projection. So a trim tab is a small flap used to balance the aircraft in flight.
Why Pilots Care
Properly set trim tabs reduce control pressures, decrease pilot workload, and allow precise hands-off flight.
Intuition Check
“Trim” here does not mean decoration or making something look neat. It means adjusting the airplane so it no longer requires constant control pressure to hold the desired flight condition.
Example Sentence 1
Once established in cruise, the pilot adjusted the elevator trim tab to relieve the back pressure on the yoke.
Example Sentence 2
With the rudder trim tabs set for the cruise power setting, the airplane flew straight with no left or right control input.