forebrain \ˈ ̷ ̷ˌ ̷ ̷\noun Etymology:fore + brain 1. a.: the anterior of the three primary divisions of the developing vertebrate brain b.: the part of the brain of the adult, comprising the telencephalon and diencephalon, that develops from the embryonic forebrain c.: telencephalon 2.: the protocerebrum of an invertebrate