Spin Axis
The tilt of the ball's spin that makes it curve.
What it is
Spin axis is the tilt of the ball's axis of rotation. A right-tilted axis (for a right-handed golfer) curves the ball to the right (a fade or slice); a left-tilted axis curves it left (a draw or hook). It's the modern, more accurate way to describe what golfers used to call 'side spin'.
Why it matters
Spin axis is what you see as curve. The flatter (more level) the axis, the straighter the shot; the more it tilts, the more the ball bends.
How to improve it
Because spin axis is driven by face to path, the way to flatten it is to better match your face angle and club path. Centered contact also reduces unwanted tilt.