Plain English Meaning

The airplane suddenly spins sideways on the runway during landing or taxi, pivoting like a shopping cart whose back end has swung out in front of the front end.

Full Definition

An uncontrolled, rapid horizontal rotation of an aircraft about its vertical axis while on the ground, typically during landing rollout or taxi. It is most common in tailwheel (conventional gear) aircraft, where the center of gravity is behind the main wheels, making any sideways swerve tend to amplify rather than self-correct.