Definition
A horizontal stabilizer at the tail of an aircraft whose angle of incidence can be changed in flight to trim the aircraft for a desired pitch attitude without holding continuous control pressure on the elevator. The pilot adjusts it through a trim wheel or switch, which repositions the entire stabilizer surface so the aerodynamic load on the elevator is neutralized at the chosen airspeed and configuration.
Plain English
The whole tail surface tilts a little when you trim, instead of just a small tab on it. This takes the pressure off the controls so you do not have to keep pushing or pulling on the yoke to hold the nose where you want it.
Context Anchor
Seen in trim discussions, especially when learning how pitch trim reduces control pressure during climbs, cruise, descents, and approaches.
Derivation
"Stabilizer" comes from Latin stabilis, meaning steady or fixed. The horizontal stabilizer is the tail surface that keeps the aircraft steady in pitch. "Adjustable" simply means its angle can be changed in flight, so the surface that normally just stabilizes the aircraft can also be moved to set a new trimmed pitch attitude.
Why Pilots Care
Reduces sustained control forces during instrument flight and allows precise attitude maintenance with minimal pilot input.
Intuition Check
Adjustable does not mean the stabilizer is only set by mechanics on the ground. In this context, it can mean a stabilizer the pilot can reposition with the trim system to reduce control pressure in flight.
Example Sentence 1
After leveling off in cruise, the pilot rolled the trim wheel forward and the adjustable stabilizer relieved the back pressure on the yoke.
Example Sentence 2
In cruise, the adjustable stabilizer was set slightly nose-up to maintain level flight hands-off.