The Collaborative International Dictionary of English v.0.48:

Diencephalon \Di`en*ceph"a*lon\, n. [NL. See Dia-, and
   Encephalon.] (Anat.)
   The interbrain or thalamencephalon; -- sometimes abbreviated
   to dien. See Thalamencephalon.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diencephalon
    n 1: the posterior division of the forebrain; connects the
         cerebral hemispheres with the mesencephalon [syn:
         diencephalon, interbrain, betweenbrain,
         thalmencephalon]