Plain English Meaning
A shudder or shaking you feel through the airframe and controls when the airflow over the wing starts breaking up. It's an early warning that the wing is about to stop flying.
Full Definition
A rapid, irregular shaking or vibration of the airframe and flight controls caused by turbulent, separated airflow striking the aircraft's structure — most commonly the tail surfaces — as the wing approaches or enters a stalled condition.
Context Anchor
A pilot may feel buffeting during stall practice, during slow flight near a stall, or anytime the airplane is flying with disturbed airflow around the or tail.