- the middle of the three primary divisions of the brain in the embryo of a vertebrate or the part of the adult brain derived from this tissue; mesencephalon.
- the nontechnical name for mesencephalon
- The middle part of the vertebrate brain. In most animals except mammals, the midbrain processes sensory information. In mammals, it serves primarily to connect the forebrain with the hindbrain. Compare forebrain hindbrain.