Mesial plane

from The Collaborative International Dictionary of English v.0.48

Mesial \Me"sial\ (?; 277), a. [Gr. me`sos middle.] (Anat.)
   Middle; median; in, or in the region of, the mesial plane;
   internal; -- opposed to {lateral}.
   [1913 Webster]

   {Mesial plane}. (Anat.) See {Meson}.
      [1913 Webster]