Plain English Meaning
How far a control surface like the elevator is moved away from its centered position when the pilot moves the stick or yoke. A small movement equals a small deflection; a large movement equals a large deflection.
Full Definition
The angular displacement of the elevator (or other control surface) from its neutral, streamlined position, produced when the pilot moves the corresponding cockpit control. The amount of deflection determines how much the airflow over the surface is altered, which in turn determines the size of the aerodynamic force generated to pitch, roll, or the .