In aerodynamics, the critical Mach Number (Mcr or Mcrit) of an aircraft is the lowest Mach number at which the airflow over any part of the aircraft reaches the speed of sound.
For all aircraft in flight, the speed of the airflow around the aircraft is not exactly the same as the airspeed of the aircraft due to the airflow speeding up and slowing down to travel around the aircraft structure.
At the Critical Mach number, local airflow near some areas of the airframe reaches the speed of sound, even though the aircraft itself has an airspeed lower than Mach 1.0. This creates a weak shock wave. In aircraft not designed for transonic or supersonic flight, speeds greater than the Critical Mach number will cause the drag coefficient to increase suddenly causing a dramatic increase in total drag and changes to the airflow over the flight control surfaces will lead to deterioration in control of the aircraft.
In aircraft not designed to fly at the Critical Mach number, shock waves in the flow over the wing and tailplane can be sufficient to stall the wing, make control surfaces ineffective, or lead to loss of control.