Plain English Meaning
The pilot is flying the airplane normally and everything is working, but they fly it into the ground, water, or something on the ground without realizing they are about to hit it.
Full Definition
An accident category in which an airworthy aircraft, under the full control of the pilot, is unintentionally flown into the ground, water, or an obstacle. The flight path is being actively managed by the pilot at the moment of impact — the aircraft is not malfunctioning and is not out of control — but the pilot is unaware that the current path will result in a collision with terrain.
Context Anchor
Seen in accident prevention, energy management, , - flying, and any of maintaining safe height above the ground.