Plain English Meaning
The engine quits or loses enough power to no longer sustain flight shortly after the airplane has left the runway and started climbing.
Full Definition
A loss of engine power that occurs during the initial climb after liftoff, while the airplane is at low altitude with limited airspeed, limited options for landing areas, and limited time to react. It is one of the most critical emergencies in single-engine flying because the airplane has departed the runway environment but is not yet high enough to glide or freely.