Plain English Meaning

It is the turn you make at the end of the leg flown with the wind behind you, onto the next leg where the wind hits you from the side. Because the wind is pushing you fastest and trying to carry you wide, this turn needs the steepest bank of the four corners.

Full Definition

The third turn flown in a rectangular traffic pattern, transitioning the airplane from the downwind leg (flown parallel to and in the same direction as the wind) onto the crosswind leg (flown perpendicular to the wind). Because the airplane begins this turn with a tailwind and finishes it with a direct crosswind, the groundspeed is highest at the start of the turn and the wind tends to push the airplane away from the field. To track a square ground path, the pilot must use the steepest bank of the four pattern turns and with a into the wind on the new leg.