Source : Webster's Revised Unabridged Dictionary (1913)

Meg- \Meg-\, Mega \Meg"a\, Megalo- \Meg"a*lo-\ [Gr. me`gas, gen.
   mega`loy, great.]
   Combining forms signifying:
   (a) Great, extended, powerful; as, megascope, megacosm.
   (b) (Metric System, Elec., Mech., etc.) A million times, a
       million of; as, megameter, a million meters; megafarad, a
       million farads; megohm, a million ohms.

Source : Free On-Line Dictionary of Computing

mega-
     
        {prefix}