Definition of: diencephalon

(dī′en·sef′ə·lon) noun Anat.
That part of the brain forming the posterior part of the prosencephalon, from which are developed the pineal body, the pituitary, and other structures of the third ventricle; the interbrain or middle brain: also called thalamencephalon. [<NL <Gk. di(a)- between + enkephalos brain]