The balance point of the clubhead — the single spot where all its mass effectively acts. Where designers put it changes ball flight: a low, rearward CG launches the ball higher and makes the head more stable on mishits (great for most golfers), while a forward CG reduces spin for a flatter, more workable flight that better players chase. Moving the CG even a few millimeters measurably changes launch and spin, which is why adjustable drivers put sliding weights on the sole — you're literally relocating the CG.