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

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

MEGA
       MessdatenErfassung und Graphische Auswertung


The Jargon File (version 4.4.7, 29 Dec 2003):

mega-
 /me'g@/, pref.

    [SI] See quantifiers.


The Free On-line Dictionary of Computing (30 December 2018):

prefix
atto-
exa-
femto-
giga-
kilo-
mega-
micro-
milli-
peta-
pico-
tera-
yocto-
yotta-
zepto
zetta-

   1.  An SI prefix used to multiply the value of an SI
   (Système International) unit by some power of ten.

   2.  In mathematics or programming, a
   prefix operator is one that is written before its operand.  In
   a programming language using prefix notation, all operators
   are prefix operators.

   (2014-07-08)