Definition
A state of flight in which the aircraft's trim controls have been adjusted so that the aircraft maintains a desired attitude and airspeed without the pilot needing to apply continuous pressure to the flight controls.
Plain English
The airplane is set up so it flies at the speed and attitude you want on its own, without you having to push or pull on the controls to hold it there.
Context Anchor
Seen in instrument flying and speed stability discussions, especially when describing how the airplane reacts after being disturbed from its selected speed.
Derivation
From nautical 'trim,' meaning to balance a vessel by adjusting its load. In aircraft, trimming balances aerodynamic forces on the controls so the aircraft holds its setting without pilot pressure.
Why Pilots Care
A properly trimmed condition reduces pilot workload and lets the aircraft naturally return to the chosen speed after small disturbances.
Intuition Check
Trimmed does not mean the airplane is set forever or safe to ignore. It means it is balanced for the current speed, power, and configuration; when those change, the trimmed condition may change too.
Example Sentence 1
Once established at cruise airspeed, the pilot adjusted the elevator trim until the aircraft was in a trimmed condition and the control pressures were neutral.
Example Sentence 2
In a trimmed condition the airplane tends to return to its set speed if a gust momentarily changes the airspeed.