Dictionary.com
definitions

meso-

  1. a combining form meaning “middle,” used in the formation of compound words: mesocephalic.
Also especially before a vowel, mes-.

Origin of meso-

combining form representing Greek mésos middle, in the middle; akin to Latin medius; see mid1
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for meso-

meso-

before a vowel mes-

combining form
  1. middle or intermediatemesomorph

Word Origin for meso-

from Greek misos middle
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Word Origin and History for meso-

word-forming element meaning "middle, intermediate, halfway," sometimes mes- before vowels, comb. form of Greek mesos "middle, in the middle; middling, moderate; between" (see medial (adj.)).

Online Etymology Dictionary, © 2010 Douglas Harper
meso- in Medicine

meso-

pref.
  1. In the middle; middle:mesoderm.
  2. Intermediate:mesomere.
  3. Mesentery:mesoileum.
  4. Characterizing a superimposable isomer:meso-2,3-dichlorobutane.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.