The Collaborative International Dictionary of English v.0.48:

Cerebellum \Cer`e*bel"lum\, n.; pl. E. Cerebellums, L.
   Cerebella. [L., dim. of cerebrum brain.] (Anat.)
   The large lobe of the hind brain in front of and above the
   medulla; the little brain. It controls combined muscular
   action. See Brain.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cerebellum
    n 1: a major division of the vertebrate brain; situated above
         the medulla oblongata and beneath the cerebrum in humans