Plain English Meaning
When an airplane suddenly spins sideways on the runway while rolling on its wheels, swinging around like the hand of a clock instead of tracking straight.
Full Definition
An uncontrolled, rapid horizontal rotation of an airplane about its vertical axis while on the ground, typically occurring during the landing roll or takeoff roll. It happens when the tail of the aircraft swings outward faster than the pilot can correct, causing the airplane to pivot sharply, often resulting in a wingtip strike, gear damage, or loss of directional control.