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 American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.