Plain English Meaning
The speed at which an aircraft starts to feel a sudden jump in drag and other forces because shock waves are forming on the wing. It happens just past the speed where airflow first goes supersonic, and it marks the start of real handling and performance changes.
Full Definition
The Mach number at which the aerodynamic forces on an airfoil — particularly drag — begin to rise sharply due to the formation of shock waves on the wing surface. It occurs slightly above the critical Mach number and marks the point where the airflow disturbances from supersonic regions become strong enough to significantly alter lift, drag, and pitching moment.