Downswing
The motion from the top of the backswing down through impact.
It only lasts about a quarter of a second for most good players, yet it's where the vast majority of clubhead speed actually gets created, the backswing simply sets up the position to unleash it from.
See the related guide: Gaining swing speed
A full explanation, in plain language.
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.
Impact
The moment the clubface actually contacts the ball, where clubface angle, path, and attack angle all come together to decide the shot.