The Collaborative International Dictionary of English v.0.48:

Epencephalon \Ep`en*ceph"a*lon\, n. [NL., fr. Gr. 'epi` upon,
   near + ? brain.] (Anat.)
   The segment of the brain next behind the midbrain, including
   the cerebellum and pons; the hindbrain. Sometimes abbreviated
   to epen.
   [1913 Webster]