The interbrain or thalamencephalon; -- sometimes abbreviated to dien. See Thalamencephalon.

The portion of the brain that contains two of the most critical areas of the brain, the thalamus and hypothalamus.

The back of the forebrain that connects the midbrain with the cerebral hemispheres, encloses the third ventricle, and contains the thalamus and hypothalamus. Also called the interbrain. Reference: B1

Greek dia = between, and enkephalos = brain, hence in general the structures surrounding the 3rd ventricle. adjective - diencephalic.