Dictionary.com's Word of the Year is...
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.)).
meso- or mes-pref.
In the middle; middle: mesoderm.
Characterizing a superimposable isomer: meso-2,3-dichlorobutane.