Definition
An aerodynamic condition in which the wing's angle of attack exceeds its critical angle, causing the smooth airflow over the upper surface to separate and lift to decrease sharply. A stall can occur at any airspeed, in any attitude, and at any power setting.
Plain English
A stall is what happens when a wing is tilted too steeply into the oncoming air. The air can no longer flow smoothly over the top of the wing, so the wing stops producing enough lift to hold the airplane up.
Context Anchor
You encounter this term in stall awareness, slow flight, takeoff and landing practice, and any discussion of controlling the airplane near its low-speed handling limits.
Derivation
From the Old English 'steall,' meaning a fixed position or standstill -- the same root as a market stall or a horse stall. In aviation it does not mean the engine has stopped or that the airplane has stopped moving. It means the wing has stopped flying, even though the airplane may still be moving fast.
Why Pilots Care
An unrecognized stall can cause an abrupt loss of altitude and control, especially near the ground.
Analogy
Hold your hand out of a moving car window and tilt it slightly upward; it gets a smooth lifting push. Tilt it too far, and the smooth push breaks down and your hand buffets instead. A wing behaves in a similar way when its angle to the airflow becomes too large.
Grounding Statement
A stall is about the wing’s angle to the airflow, not simply about flying slowly.
Intuition Check
Do not read stall as an engine quitting or the airplane stopping in the air. In this context, stall means the wing has exceeded its lift-producing angle and airflow has separated from it.
Example Sentence 1
The instructor demonstrated a power-off stall by slowly raising the nose until the stall warning horn sounded and the wing stalled.
Example Sentence 2
During the power-off stall demonstration the airplane entered a stall with the nose high and power at idle.