Definition
The condition in which an airplane maintains a desired attitude and flight path without the pilot having to hold continuous pressure on the flight controls. Trim is achieved by adjusting trim devices (such as trim tabs on the elevator, rudder, and sometimes ailerons) so that aerodynamic forces hold the control surfaces in the position the pilot wants, relieving control pressures.
Plain English
Trim is the act of adjusting small control settings so the airplane flies hands-off in the attitude you want, without you having to push or pull on the controls to hold it there.
Context Anchor
In instrument attitude flying, airplane trim is used after setting pitch and power for level flight, a climb, or a descent so the airplane stays steady while the pilot watches the instruments.
Derivation
From the nautical term 'to trim,' meaning to adjust a ship's sails or balance for steady, efficient sailing. Carried into aviation with the same idea: balance the aircraft so it flies steady on its own.
Why Pilots Care
Proper trim reduces pilot workload and fatigue, especially important during instrument flight where attention must stay on the instruments.
Analogy
Similar to setting cruise control in a car so the vehicle maintains speed without continuous pressure on the accelerator.
Intuition Check
Trim does not mean making the airplane neat or perfect. Here it means adjusting the airplane so the controls are balanced and the pilot does not have to hold constant force.
Example Sentence 1
After leveling off at cruise altitude, the pilot adjusted pitch trim until the airplane held altitude with no back or forward pressure on the yoke.
Example Sentence 2
Proper use of the trim wheel allowed the pilot to maintain altitude hands-off while scanning the instruments.