Buffet is an aerodynamic phenomenon . It usually occurs in the transonic speed range (the range in which the approach velocity is still subsonic , but is already supersonic on the wing ). When buffet occurs, the otherwise steady flow turns into an unsteady state - in other words, it changes over time and turns into periodic oscillations.
Buffet is triggered by the interaction of a compression shock and a detached turbulent boundary layer behind the shock. In the transonic speed range at least one local supersonic area occurs, at the end of which (downstream) there is a shock wave. If the angle of attack is increased so much that a local (not too small) boundary layer separation takes place behind the impact , then their interaction leads to buffet. The onset of the buffet occurs without external stimulation from a certain buoyancy value, amplifies itself through self-excited vibrations and stops again by itself above a certain angle of attack / buoyancy value.
In practice, Buffet looks like this: From a certain local separation length, the shock wave is pushed upstream towards the leading edge of the wing. This changes its impact strength (the pressure ratio between before and after the impact). The (positive) pressure gradient after the impact, however, depends on this and directly determines the length of the separation. The separation becomes smaller and allows the shock wave to come back downstream. But that in turn increases the separation length and pushes the joint forward again and the circle is closed.
The shock wave travels back and forth periodically and the flow separation from the wing also periodically changes its length. Since the impact strength directly influences the aerodynamic drag and the separation length the lift , these periodic vibrations are induced in the wing structure. The structural answer of the aircraft structure to buffet is called buffeting and can have serious consequences for the structure and the flight condition.