Plain English Meaning
A crash where the airplane is working perfectly and the pilot is flying it normally — but they don't realize they're about to hit something until it's too late.
Full Definition
An accident in which a fully airworthy aircraft, under the control of a qualified pilot, is unintentionally flown into the ground, water, or an obstacle. The aircraft is operating normally and responding to control inputs at the moment of impact; the pilot is simply unaware that a collision is imminent.
Context Anchor
Seen in accident reports, safety training, approach planning, night flying, and flight in poor or near rising ground.