Plain English Meaning
How far the rudder is pushed left or right by the pilot's feet on the pedals. A small push gives a small movement; a firm push gives a larger one. This swings the nose of the airplane in the direction of the pedal that is being pressed.
Full Definition
Movements of the rudder away from its neutral (centered) position, produced by the pilot pressing the left or right rudder pedal. Each press pivots the rudder to one side of the vertical stabilizer, creating a sideways aerodynamic force at the tail that yaws the airplane's nose left or right. The amount of deflection refers to how far the rudder is moved from center, and is controlled by how much pressure the pilot applies to the pedal.
Context Anchor
Encountered during the takeoff roll, when the pilot is using the to keep the airplane straight down the .