Plain English Meaning
A shudder or shaking you feel through the airframe when the air stops flowing smoothly over the wing. It's an early physical warning that the airplane is getting close to a stall.
Full Definition
A vibration or shaking of the airframe caused by turbulent, separated airflow striking the wings, tail, or fuselage. In slow flight and stall situations, buffet is an aerodynamic warning that the wing is approaching its critical angle of attack and airflow is beginning to break away from the upper wing surface.
Context Anchor
Seen in slow flight and training, when a is to recognize the physical that the wing is close to losing smooth airflow.