Impact
The moment the clubface actually contacts the ball, where clubface angle, path, and attack angle all come together to decide the shot.
It happens fast: per USGA research, the ball is only in contact with the clubface for about half a millisecond, in a collision producing roughly 3,000 pounds of force. Nothing your hands do during that half-millisecond changes the shot, whatever it's going to do was already decided by the swing that led into it.
More swing mechanics terms
Address
The setup position a golfer takes before starting the swing.
Takeaway
The first foot or two of the backswing, as the club first moves away from the ball.
Backswing
The motion from address to the top, where the club is loaded before the downswing.
Top of the Backswing
The highest, furthest point of the backswing, right before the downswing begins.
Transition
The change of direction from backswing to downswing.
Downswing
The motion from the top of the backswing down through impact.