Mach 1 is, strangely enough, a Mach number.
At any given set of conditions, it'll have TAS, CAS, EAS and IAS values - the relationship will keep moving, and is a function of temperature and density.
Mach number is an issue in light and microlight aeroplanes - engine speed and propeller diameter, plus air temperature define tip Mach number. Vibration and loss of efficiency can become problematic above about 0.8M.
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