Definition
A cockpit control, shaped like a wheel and turned by hand, that adjusts the airplane's trim system to relieve continuous control pressure the pilot would otherwise need to hold on the yoke or stick. Most commonly it controls pitch trim by repositioning the trim tab on the elevator (or moving a trimmable horizontal stabilizer), allowing the airplane to maintain a desired pitch attitude or airspeed hands-off.
Plain English
A small wheel in the cockpit that the pilot rolls forward or backward to take the constant push or pull off the controls, so the airplane will fly steadily on its own without the pilot holding pressure.
Context Anchor
Seen during the cockpit portion of a preflight check, before takeoff, and anytime the pilot adjusts control feel in flight.
Derivation
Trim' comes from the old nautical sense of adjusting a ship so it sits balanced in the water. In an airplane, trimming balances the aerodynamic forces so the airplane sits steady in the air without the pilot's help.
Why Pilots Care
Correct trim reduces pilot workload, prevents fatigue, and allows stable hands-off flight at the desired airspeed and attitude.
Intuition Check
Trim does not mean decoration or cutting something here. It means adjusting the airplane so it is balanced and easier to hold in the desired nose position.
Example Sentence 1
After leveling off at cruise altitude and setting cruise power, the pilot rolled the trim wheel forward until the airplane held altitude with no pressure on the yoke.
Example Sentence 2
After leveling off in cruise, the pilot turned the trim wheel until the yoke felt neutral in pitch.