Backswing
The motion from address to the top, where the club is loaded before the downswing.
Good tempo means the backswing takes roughly three times as long as the downswing, not because it should feel slow, but because the downswing is the fast, releasing half of the same motion.
See the related guide: Golf swing tempo, explained
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.
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.
Impact
The moment the clubface actually contacts the ball, where clubface angle, path, and attack angle all come together to decide the shot.