Source : Webster's Revised Unabridged Dictionary (1913)

Mesencephalon \Mes`en*ceph"a*lon\, n. [NL. See {Meso-} and
   {Encephalon}.] (Anat.)
   The middle segment of the brain; the midbrain. Sometimes
   abbreviated to {mesen}. See {Brain}.

Source : WordNet®

mesencephalon
     n : the middle portion of the brain [syn: {midbrain}]