When the local flow velocity is equal to the speed of the light (the speed at which the disturbance propagates) is know as the sonic speed.
Mach number for the sonic speed is 1.
It is also known as the disturbance propagates speed through the medium.
You hear a sonic boom (a blast) from a jet plane when it crosses the sound barrier; basically, traveling faster than the speed of sound; one can also see what appears like smoke, surrounding the rear of the aircraft when it approaches the speed of sound.
The terms - sub-sonic, super-sonic, hyper-sonic, are all written or spoken with reference to the speed of an object (usually jet planes) traveling close to or beyond the speed of sound.
Subsonic Speed - Slower than the speed of sound — less than about Mach 0.8.
Transonic Speed - At this speed, an aircraft is approaching the speed of sound but hasn’t yet reached and surpassed Mach 1. At some places on the aircraft the speed will exceed Mach 1, while at others it will be less than Mach 1.
Supersonic Speed - The speed of sound, which is 767.269 mph, otherwise known as Mach 1.
Hypersonic Speed - Greatly exceeds supersonic speed, starting at speeds at Mach 5 and above.