Plain English Meaning

If a wing drops and the pilot does nothing, the airplane will not pick the wing back up on its own. Instead, the bank slowly gets steeper, the nose drops, and the airplane winds itself into a tightening descending turn that keeps getting faster.

Full Definition

A condition in which an airplane, after being disturbed in roll, tends to continue rolling and enter a progressively tightening descending turn rather than returning to wings-level flight. It results from strong directional (yaw) stability combined with weak lateral (roll) stability: the airplane yaws into the bank, the outer wing travels faster and produces more lift, the bank steepens, the nose drops, and airspeed and rate of descent increase if the pilot does not intervene.