Plain English Meaning
How much runway you need to speed up for takeoff and then stop again if you decide to abort. It is the worst-case distance: full acceleration up to the decision point, then full braking to a halt.
Full Definition
The total runway distance required for an airplane to accelerate from a standing start to a specified speed (typically the takeoff decision speed or liftoff speed), and then, assuming a rejected takeoff at that speed, to bring the airplane to a complete stop on the remaining runway.
Context Anchor
Pilots encounter this term when checking takeoff performance, especially when deciding whether a runway is long enough for the airplane, weight, weather, and field conditions.