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

WordNet (r) 3.0 (2006):

meg
    n 1: the number that is represented as a one followed by 6 zeros
         [syn: million, 1000000, one thousand thousand, meg]

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

MEG
       Mega Evil Grin (slang, Usenet, IRC)


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

meg
 /meg/, n.

    See quantifiers.


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

megabyte
meg

    (MB, colloquially "meg") A Unit of data equal to
   one million bytes but see binary prefix for other definitions.
   A megabyte is 1000^2 bytes or 1000 kilobytes.

   The text of a six hundred page paperback book stored as ASCII
   characters contains about one megabyte of data.  The complete
   King James bible is 5.2 megabytes.

   1000 megabytes are one gigabyte.

   See prefix.

   (2013-11-04)