Plain English Meaning
A turn where the airplane is sliding sideways toward the outside of the curve because the pilot is using too much rudder for the amount of bank. The tail is being pushed around faster than the bank alone would turn the airplane.
Full Definition
A turn in which the rate of yaw is too great for the angle of bank, causing the airplane to slide sideways toward the outside of the turn. The ball in the inclinometer moves to the outside of the turn, indicating excessive rudder is being applied in the direction of the turn relative to the bank angle.
Context Anchor
Commonly discussed when learning about cross-control stalls, especially during an overshoot from base leg to final approach.