Plain English Meaning
How well the aircraft will actually take off on a given day — how much runway it needs to leave the ground, and how much it needs to climb safely past obstacles at the end of the runway.
Full Definition
The measurable behavior of an aircraft during the takeoff phase, expressed primarily as the ground roll distance required to lift off and the total distance required to clear a 50-foot obstacle, under specified conditions of weight, altitude, temperature, wind, runway surface, and aircraft configuration.