Spin axis is the tilt of the rotational axis of the ball relative to the horizon, measured in degrees. A spin axis tilted to the right (positive) curves the ball right for a right-handed golfer; tilted to the left (negative), the ball curves left. A spin axis at zero degrees produces a straight ball flight.
It's the primary cause of curvature in a shot — separate from the spin rate, which governs how much the ball climbs and how much it stops on the green.
Why it matters
Spin axis is what turns a slice into a slice and a draw into a draw. Two shots with the same spin rate can fly very differently if their spin axes tilt in opposite directions.
For practice, working on spin axis means working on the relationship between club path and face angle at impact — the two factors that combine to tilt the ball's spin one way or the other.
How launch monitors compute it
Launch monitors typically derive spin axis from a combination of ball flight curvature (radar) or measured spin and direction at impact (photometric). Some monitors report it explicitly in degrees; others render it visually as the curve of the ball flight.