Maximum speed for stability characteristics, V FC /M FC

a speed that may not be less than a speed midway between maximum operating limit speed (V MO /M MO ) and demonstrated flight diving speed (V DF /M DF ), except that, for altitudes where the Mach number is the limiting factor, M FC need not exceed the Mach number at which effective speed warning occurs.

Source: 14 CFR § 1.1