Mach number

Summary

The Mach number $\mathrm{M} = u / c$ is the ratio of the flow speed to the local speed of sound. It measures how important the compressibility of the fluid is. Flows are called subsonic for $\mathrm{M} < 1$ and supersonic for $\mathrm{M} > 1$, and the two regimes are governed by equations of different type.