Plain English Meaning
How much distance you need to take off and climb to 50 feet if one engine quits partway through the takeoff roll and you decide to keep going on the engine you have left.
Full Definition
The total runway and air distance required for a multi-engine airplane to accelerate from a standing start to takeoff decision speed, experience the failure of the critical engine at that point, and then continue the takeoff on the remaining engine(s) to a height of 50 feet above the runway surface.
Context Anchor
Seen in takeoff performance planning, especially for multiengine airplanes and runway-length decisions.