Source : Webster's Revised Unabridged Dictionary (1913)

Forebrain \Fore"brain`\, n. (Anat.)
   The anterior of the three principal divisions of the brain,
   including the prosencephalon and thalamencephalon. Sometimes
   restricted to the prosencephalon only. See {Brain}.

Source : WordNet®

forebrain
     n : the anterior portion of the brain; the part of the brain
         that develops from the anterior part of the neural tube
         [syn: {prosencephalon}]