Plain English Meaning
How quickly the aircraft is speeding up on the takeoff roll or slowing down on the landing roll. A high rate means it gains or loses speed quickly. A low rate means it takes longer to reach takeoff speed or come to a stop.
Full Definition
The change in an aircraft's speed over time during the takeoff or landing roll. Acceleration is the rate at which speed increases as the aircraft gains velocity for liftoff; deceleration is the rate at which speed decreases as the aircraft slows after touchdown or during a rejected takeoff. Both rates are influenced by thrust, drag, braking, runway surface, slope, , , and .