Plain English Meaning
It's the spot on downwind, level with where you plan to land, where you pull the power to idle and start the glide down to the runway. From here on, you're flying the airplane to the touchdown point using only what altitude and energy you already have.
Full Definition
The point on the downwind leg of the traffic pattern, abeam the intended touchdown point, at which the pilot reduces power to idle to begin a 180° power-off approach. From this position, the pilot judges altitude, distance, and wind to plan the gliding turn through to without adding .