Source : Webster's Revised Unabridged Dictionary (1913)

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

   {Mesial plane}. (Anat.) See {Meson}.

Source : WordNet®

mesial
     adj : being in or directed toward the midline or mesial plane of
           the body [ant: {distal}]