Plain English Meaning
The engine is at idle and not producing useful thrust, so the airplane is gliding or descending on its own energy.
Full Definition
A flight condition in which the engine is producing no thrust, with the throttle fully closed (at idle) so that the aircraft is operating purely on the energy from its airspeed and altitude. Power-off performance figures, such as power-off stall speeds, are measured under this condition.
Context Anchor
Seen in stall speed performance charts and stall practice, where power setting changes how the airplane behaves near a stall.
Why Pilots Care
Power-off stall speeds are used to determine minimum safe speeds for and .