The Collaborative International Dictionary of English v.0.48:

Thalamencephalon \Thal`a*men*ceph"a*lon\, n. [NL. See
   Thalamus, and Encephalon.] (Anat.)
   The segment of the brain next in front of the midbrain,
   including the thalami, pineal gland, and pituitary body; the
   diencephalon; the interbrain.
   [1913 Webster]