Plain English Meaning
The part of the flight where the airplane speeds up on the runway, lifts off the ground, and climbs away from the airport until it reaches a safe height and steady climb.
Full Definition
The phase of flight that begins with the airplane accelerating along the runway under takeoff power and continues through liftoff, initial climb, and the establishment of a stable climb attitude and airspeed at a safe altitude. It encompasses the transition from ground operation to sustained flight, including rotation, lift-off, and the climbout to a safe maneuvering altitude.