Plain English Meaning
The first part of climbing after takeoff. It begins the moment the wheels leave the ground and ends once the airplane is climbing steadily at a safe height above the ground.
Full Definition
The phase of flight from the moment the airplane lifts off the runway until it reaches a safe maneuvering altitude, typically at least 500 feet above ground level, where a positive rate of climb has been established and the airplane has been configured for sustained climb (gear and flaps retracted as appropriate, climb airspeed established).
Context Anchor
You encounter this term in takeoff training, especially when discussing what to do immediately after and before settling into the rest of the .