Tempo
The overall rhythm and speed ratio between the backswing and downswing.
Good tempo doesn't mean slow, it means a consistent ratio (roughly 3:1 backswing-to-downswing time for most good players) that's repeatable under pressure.
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.
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.