Plain English Meaning

It is the airplane's speed at which the air flowing over part of the wing first goes supersonic, while the airplane is still flying below the speed of sound. Past this point, shock waves start forming on the wing and the airplane begins to behave differently.

Full Definition

The critical Mach number (MCRIT) is the speed, expressed as a fraction of the speed of sound, at which airflow over some part of an airplane (typically the upper wing surface) first reaches the speed of sound, even though the airplane itself is still flying slower than the speed of sound. Flight at or above this speed produces localized supersonic flow and the shock waves associated with it, leading to increased drag, buffet, and changes in handling.