forebrain

1. The most anterior of the three primary regions of the embryonic brain from which the telencephalon and diencephalon develop.
2. The segment of the adult brain that develops from the embryonic forebrain and includes the cerebrum, thalamus, and hypothalamus.

The anterior part of the brain that gives rise to the cerebral hemispheres, olfactory lobes (concerned with the sense of smell), pituitary gland, pineal gland, and the optic chiasma that carries nerve fibres from the eyes to the brain.

The prosencephalon.

The noun has one meaning:

Meaning #1: the anterior portion of the brain; the part of the brain that develops from the anterior part of the neural tube
  Synonym: prosencephalon