Definition
A trim system that relieves a constant sideways (lateral) control pressure on the ailerons, allowing the airplane to maintain a wings-level attitude or a desired bank without the pilot having to hold pressure on the control wheel or stick.
Plain English
A small adjustment that takes away the steady left or right pressure you'd otherwise have to hold on the controls to keep the wings where you want them.
Context Anchor
You encounter aileron trim when adjusting the airplane after takeoff, in cruise, or any time the airplane keeps wanting to bank left or right even though you want it to stay level.
Derivation
‘Aileron’ comes from French for ‘little wing’ — the hinged surfaces on the wings that roll the airplane. ‘Trim’ here means a fine adjustment that holds a setting, like trimming a sail. Together: a fine adjustment for the rolling controls.
Why Pilots Care
It reduces pilot workload and fatigue by removing the need for continuous yoke pressure to hold wings level.
Intuition Check
Trim does not mean cutting something off or making it look neat here. In this context, trim means making a small control adjustment so the airplane stays balanced without constant hand pressure.
Example Sentence 1
After burning fuel from one tank longer than the other, the pilot used aileron trim to relieve the left-wing-low pressure during cruise.
Example Sentence 2
During cruise the pilot fine-tuned the aileron trim to compensate for a slight fuel imbalance between the wings.