See diencephalon in American Heritage Dictionary 4
(dīˌĕn-sĕfˈə-lŏnˌ, -lən)
noun
The posterior part of the forebrain that connects the midbrain with the cerebral hemispheres, encloses the third ventricle, and contains the thalamus and hypothalamus. Also called betweenbrain, interbrain, thalamencephalon.