The angle between the shaft and the ground when the club sits flat on its sole. If it doesn't match your body and swing, the face points off-line at impact even on a perfect strike: too upright (toe up) sends shots left for a right-hander, too flat (heel up) sends them right. The effect grows with loft, so a wedge that's 2° off misses by more than a driver. This is why a proper fitting checks lie with impact tape or strike boards — a bent iron is a cheap fix for a chronic pull or push.