Plain English Meaning
The airplane has slowed down so much, so close to the ground, that nothing the pilot does will speed it back up in time. A crash or hard landing is now unavoidable.
Full Definition
A loss of airspeed and energy from which the airplane cannot recover before ground contact, even with full power and correct control inputs. It is the point at which the available altitude, thrust, and time are no longer sufficient to arrest the descent or rebuild flying speed.
Context Anchor
Seen in energy management and low-speed flight discussions, especially when an airplane is close to the ground, climbing too steeply, or being flown too slowly.