Plain English Meaning

It is the small wing at the tail that can tilt up or down as a whole. By tilting the whole tail surface, the pilot trims the airplane so it flies level without having to hold pressure on the controls.

Full Definition

A horizontal tail surface whose angle of incidence can be changed in flight to trim the aircraft for varying load and speed conditions. Instead of using a fixed horizontal stabilizer with a movable trim tab on the elevator, the entire stabilizer pivots, typically driven by a , to set the of the airplane.