飞机飞行时，当随飞行速度增大，上翼面压力最低点的速度等于此点上的音速时的飞机飞行马赫数称为临界马赫数。

In aerodynamics, the critical Mach number (Mcr or M* ) of an aircraft is the lowest Mach number at which the airflow over some point of the aircraft reaches the speed of sound, but does not exceed it.

At the lower critical Mach number, airflow around the entire aircraft is subsonic. At the upper critical Mach number, airflow around the entire aircraft is supersonic.