Waggle
A small, rhythmic movement of the clubhead (and often the wrists or hips) during the pre-shot routine, just before starting the backswing.
The waggle isn't just a nervous habit, most good players use it on purpose to stay loose, rehearse the feel of the takeaway, and avoid getting stiff or "frozen" over the ball. There's no single correct waggle, the point is finding a small movement that helps you start the swing smoothly rather than jerkily.
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.