Plain English Meaning

A shaking of the aircraft that happens at high altitude, even when the airplane isn't flying particularly fast, because the air over the wing is moving fast enough to cause shock waves and disturbed airflow.

Full Definition

An airframe vibration caused by airflow separation that occurs at relatively low indicated airspeeds in high-altitude flight, when the local airflow over portions of the wing accelerates to supersonic speed even though the aircraft itself is well below its critical Mach number indication. As altitude increases, the margin between the stall speed and the speed at which begins narrows, eventually meeting at a sometimes called '.'