Critical Mach number can be defined as:

Critical Mach number refers specifically to the speed at which airflow over any part of an aircraft first reaches Mach 1.0 without exceeding it. This is a significant concept in aerodynamics as it marks the point at which an aircraft begins to experience compressibility effects, which can lead to increased drag and potential issues with control and stability. At this speed, the aircraft is still subsonic in terms of overall performance, but certain flow characteristics over the wing or fuselage are transitioning to supersonic speeds. Understanding the critical Mach number is essential for pilots and engineers, as it plays a crucial role in ensuring safe and efficient flight operations, especially for aircraft that operate near the speed of sound.