The bottom of the swing arc — the spot where the clubhead stops traveling downward and starts back up — measured relative to the ball. Launch monitors report it as a distance before or after impact, and it's really attack angle told a more vivid way: hitting down on the ball means the low point comes after the ball, which is exactly what good iron play requires — ball first, then turf, divot on the target side. When the arc bottoms out early you get the fat shot; when the ball is caught well past low point, on the upswing, you catch it thin. The one club that wants the ball after low point is the driver, teed up and struck on the rise.