"Mach number is a common 'ratio' unit of speed when one is talking about aircrafts. By definition, Mach number is a ratio of the speed of a body (aircraft) to the speed of sound in the undisturbed medium through which the body is traveling. It is said that the aircraft is flying at Mach 1 if its speed is equal to the speed of sound in air (which is 332 m/s or 1195 km/hr or 717 miles/hour.) An aircraft flying at Mach 2 is flying at twice the speed of sound in air, etc."
http://www.physlink.com/Education/askExperts/ae58.cfm
Now I am confused Mach is a ratio but Mach 1 is an actual speed?
Mach 1 is when the speed is equal to the speed of sound -- a ratio of 1.0-to-1.
okie01 is right, “mach” is a ratio of the actual speed vs. the true air speed of sound under that set of conditions (mostly changes with altitude).