Condition which arises when the smooth airflow over a wing′s upper surface breaks down and its lift is destroyed.
With a low angle of attack the airflow is smooth and attached to both the upper and lower surfaces.
As the angle of attack increases the turbulence increases and the airflow detaches from the upper surface earlier.
The last image shows a fully turbulent upper airflow and at this point lift would be lost on the wing, it would stall.
See also: Wing.