Top of the Backswing
The highest, furthest point of the backswing, right before the downswing begins.
This is also where sequencing actually starts: an efficient downswing begins with the lower body initiating from this position, before the arms and club have finished getting here.
See the related guide: Sequencing, 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.
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.