Definition: mega-

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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: The Free On-line Dictionary of Computing (2003-OCT-10) 

mega-

   prefix

Source: V.E.R.A. -- Virtual Entity of Relevant Acronyms December 2001 

MEGA
        MessdatenErfassung und Graphische Auswertung

Source: Jargon File (4.3.1, 29 Jun 2001) 

mega- /me'g*/ pref. [SI] See quantifiers.