Wind window is the 120-180 degree arc of the sky downwind of the rider in which the kite can be flown. Roughly one fourth of a sphere's surface. If the rider is facing downwind on a flat surface, like the ocean, the wind window consists of roughly all the area the rider can see, from the rider's peripheral vision on one side, along the horizon to the other side, and then directly overhead back to the first side. If the rider somehow puts the kite out of the window -- for example, by riding downwind very quickly and sending the kite directly overhead and behind -- the kite will stall and frequently fall out of the sky. (source Wikipedia)