Definition
The act of moving a trim control (such as a trim wheel, trim tab switch, or trim crank) to relieve the steady control pressure the pilot is holding on the yoke, stick, or rudder pedals, so the aircraft maintains the desired attitude with little or no effort on the primary flight controls.
Plain English
Tweaking the trim until the airplane will hold the attitude you set without you having to keep pushing or pulling on the controls.
Context Anchor
You encounter trim adjustment during instrument flying whenever you change power, speed, or airplane position and then need to reduce the pressure you feel on the controls.
Derivation
From the nautical word 'trim,' meaning to balance a vessel by adjusting its load. In aviation it carries the same idea: balancing the aerodynamic forces on the aircraft so it sits steady without the pilot fighting it.
Why Pilots Care
Correct trim reduces control forces, lowers pilot fatigue, and frees attention for instrument scan and radio work during IFR flight.
Intuition Check
Trim adjustment does not mean using trim to fly the airplane instead of the main controls. First set the airplane where you want it, then trim to relieve the pressure needed to hold it there.
Example Sentence 1
After leveling off at the assigned altitude and reducing power to cruise, she made a small trim adjustment to relieve the forward pressure on the yoke.
Example Sentence 2
During the descent the pilot used the electric trim to make a quick adjustment when airspeed began to drift.