Definition
Maximum speed for stability characteristics, V FC/MFC means a speed that may not be less than a speed midway between maximum operating limit speed (VMO/MMO) and demonstrated flight diving speed (VDF/MDF), except that, for altitudes where the Mach number is the limiting factor, MFC need not exceed the Mach number at which effective speed warning occurs.
Source: 14 CFR § 1.1 (verbatim from the Federal Aviation Regulations).