Mach number is the ratio of velocity to the local velocity of sound in the same medium.

If
Ma < 1, flow is called sub-sonic
Ma = 1, flow is called sonic
Ma > 1, flow is called super-sonic
Mach number is the ratio of velocity to the local velocity of sound in the same medium.
If
Ma < 1, flow is called sub-sonic
Ma = 1, flow is called sonic
Ma > 1, flow is called super-sonic