Whether the clubhead is traveling upward or downward as it strikes the ball, in degrees. Positive means hitting up, negative means hitting down. Irons should be negative (about −4° for a tour 7-iron) — that downward strike is what compresses the ball and makes crisp contact. The driver is the exception: because the ball sits on a tee, hitting up (+2 to +5°) launches it higher with less spin, and for an average-speed player that can be worth 15+ yards of free carry versus hitting down on it.