Early Extension
The hips and spine moving toward the ball during the downswing, standing up out of the original posture.
This is a very common fault that steepens the swing, moves the low point, and is behind a lot of thin and shanked shots.
See the related guide: Golf swing posture
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.