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

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "cerebellum":
   arachnoid, arbor vitae, archipallium, between brain, brain stem,
   cerebellar hemispheres, cerebral cortex, cerebrospinal fluid,
   cerebrum, convolution, corpus callosum, corpus striatum,
   diencephalon, dura mater, endbrain, fissure, folia, forebrain,
   fornix, frontal lobe, glial cells, globus pallidus, gray matter,
   gyrus, hindbrain, hippocampus, hypothalamus, lenticular nucleus,
   limbic lobe, little brain, lobe, mantle, medulla oblongata,
   meninges, mesencephalon, metencephalon, midbrain, myelencephalon,
   neopallium, occipital lobe, optic chiasm, pallium, parietal lobe,
   pia mater, pineal body, pituitary body, pons, reticular system,
   rhombencephalon, subthalamus, telencephalon, temporal lobe,
   thalamus, ventricle, vermis, white matter