By convention, V is a speed (expressed in knots generally in aviation).
V = veloce, vitesse, velocidad, in latin and latin languages...
And same convention, M is an expression of speed, but in Mach number.
Speeds such as Vmo - max operating speed, then Mmo, max operating Mach number.
Example for 747-200 = Vmo is 383 KIAS or Mmo is Mach .92...
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Happy contrails