Plain English Meaning

The very start of a stall — the warning stage, just before the wing actually stops flying. The aircraft is telling you it's about to stall, and a quick reduction of back pressure on the controls will prevent it.

Full Definition

The earliest stage of a stall, where the wing's angle of attack has reached the point at which airflow is just beginning to separate from the upper surface, but full aerodynamic stall has not yet developed. At this stage the pilot will typically feel buffet, hear the stall warning horn, and notice a loss of control effectiveness, and recovery can be achieved by promptly reducing the angle of attack before a full stall occurs.