The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

forebrain
    n 1: the anterior portion of the brain; the part of the brain
         that develops from the anterior part of the neural tube
         [syn: forebrain, prosencephalon]