Plain English Meaning
The part of the traffic pattern where you fly across the runway heading at right angles, just after takeoff climb-out, before turning to fly parallel to the runway in the opposite direction.
Full Definition
The leg of the airport traffic pattern flown perpendicular to the departure end of the runway, immediately after the climb-out from takeoff and before turning onto the downwind leg. In ground reference maneuvers and pattern flying, it is the segment where the airplane is flying across the wind rather than into it or with it.