When describing the speed of flow, especially supersonic flow, it is often convenient to use Mach number ($latex M$) instead of the magnitude of the velocity ($latex v$).  The Mach number is defined as the speed as multiples of the local speed of sound ($latex a$):

$latex M = \dfrac{v}{a}$

The regime of flight where $latex M < 1$ is referred to as subsonic flight while $latex M >1$ is referred to as supersonic flight.  Sonic flight is the condition where M = 1.  There is also a transonic regime for, roughly, $latex 0.8 < M < 1.2$ where in the case of a flight vehicle there may be local areas of supersonic and subsonic flow velocity.  Finally, hypersonic flight is generally defined to be $latex M > 5$ where high temperature and real gas effects need to be taken into account.

Note also that Mach number does not map directly to the same velocity.  This is because the speed of sound changes depending on, for example, atmospheric altitude due to the variable pressure and density profile in the atmosphere.  For example, at an altitude of 1 km, Mach 10 flight corresponds to 3364 m/s while at 10 km it corresponds to 2994 m/s.