The Collaborative International Dictionary of English v.0.48:

Meg- \Meg-\ (m[e^]g-), Mega \Meg"a\ (m[e^]g"[.a]-), Megalo-
\Meg"a*lo-\ (m[e^]g"[.a]*l[-o]-). [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, megabyte, a million butes; megawatt, a
       million watts; megameter, a million meters; megafarad, a
       million farads; megohm, a million ohms.
       [1913 Webster +PJC] Megacephalic

The Collaborative International Dictionary of English v.0.48:

Megalo- \Meg"a*lo-\
   See Meg-.
   [1913 Webster]