The critical Mach number (M_CRIT) is the lower limit of a speed band, known as the transonic range, in which the local Mach number may be either subsonic or supersonic. Its value varies with the angle of attack and is the free-stream Mach number at which sonic speed is reached over the upper surface of a wing. In other words, it is the speed at which the local Mach number is equal to the free-airstream Mach number. It is also the highest speed possible without supersonic flow over the wing.

The critical Mach number is inversely proportional to the angle of attack, i.e. M_CRIT decreases with increasing angle of attack and vice versa. This is because, as the angle of attack increases the local peak velocity of the airflow over the wing also increases. It therefore reaches Mach 1 sooner than it would have done at a lower angle of attack; thus, the critical Mach number is less.

At the design stage of an airplane the inclusion of some characteristics will increase the value of the critical Mach number:

A thin airfoil cross-section (low thickness to chord ratio).

Generally, a thicker wing with a large amount of camber has a lower critical Mach number than a thin wing with little camber because the airflow over its upper surface accelerates to a higher speed than it would over a thin wing. Therefore, an airfoil with reduced camber will have a higher M_CRIT.