Definition
An indication, given to the pilot before an aerodynamic stall occurs, that the wing is approaching the critical angle of attack. The warning may be aerodynamic (such as airframe buffet), or produced by a dedicated stall warning system using a horn, light, or stick shaker activated by an angle-of-attack sensor or lift detector on the wing's leading edge.
Plain English
A signal that tells the pilot the wing is getting close to the angle where it will stop producing enough lift to fly. It can be a noise, a light, a shake in the controls, or a buffet you feel through the airframe.
Context Anchor
Seen in slow flight, stall practice, takeoff and landing discussions, and anytime the airplane is flown near the low-speed edge of its normal range.
Derivation
“Stall” originally means to stop or come to a standstill. In aviation, it refers to the wing losing its normal smooth lift because the airflow is no longer staying attached properly. “Warning” means an alert before the full problem occurs.
Why Pilots Care
It gives the pilot time to lower the nose and regain lift before a full stall occurs.
Intuition Check
A stall warning is not an engine-stall warning. It is about the wing nearing an aerodynamic stall, not the engine quitting.
Example Sentence 1
As the airspeed bled off in the slow flight maneuver, the stall warning horn began to sound, and the pilot lowered the nose slightly to reduce the angle of attack.
Example Sentence 2
The pilot checked that the stall warning operated correctly during the preflight inspection.