Plain English Meaning
The instant the airplane stops rolling on the runway and starts flying. The wheels come off the ground and the airplane is now supported by the air rather than the pavement.
Full Definition
The moment during the takeoff roll when the airplane's wings produce enough lift to support its weight and the wheels leave the runway surface, transitioning the airplane from ground operation to flight.
Context Anchor
You will see this term in takeoff procedures, especially when discussing when to let the airplane leave the surface and begin the initial climb.