Plain English Meaning
The air stops flowing smoothly over the wing and instead breaks away into a messy, swirling pattern. When that happens, the wing makes less lift and more drag, and the controls feel less responsive.
Full Definition
The condition in which the smooth flow of air over a wing or other airfoil breaks away from the surface, becoming turbulent and disorganized. When this happens, the airfoil produces less lift and more drag, and control effectiveness is reduced. Airflow separation is what occurs at the point of stall, and it can also be triggered prematurely by ice, surface damage, or contamination disrupting the airfoil shape.
Context Anchor
Seen in discussions of stalls, wing and , handling, and how ice changes the way air moves over an airfoil.