Plain English Meaning
Keeping the wing meeting the air at an angle that produces enough lift to fly the airplane safely, while staying well clear of the angle where the wing would stop producing lift and stall.
Full Definition
The pilot's active control of the wing's angle of attack throughout all phases of flight to keep the airplane operating within a safe margin from the critical (stalling) angle of attack. It involves using pitch control, power, and configuration changes so that the wing always produces the lift required for the maneuver without approaching the angle at which the wing stalls.