Source : Webster's Revised Unabridged Dictionary (1913)

Diencephalon \Di`en*ceph"a*lon\, n. [NL. See {Dia-}, and
   {Encephalon}.] (Anat.)
   The interbrain or thalamencephalon; -- sometimes abbreviated
   to dien. See {Thalamencephalon}.

Source : WordNet®

diencephalon
     n : the posterior division of the forebrain; connects the
         cerebral hemispheres with the mesencephalon [syn: {interbrain},
          {betweenbrain}, {thalmencephalon}]