The Collaborative International Dictionary of English v.0.48:

Ganglion \Gan"gli*on\, n.; pl. L. Ganglia, E. Ganglions. [L.
   ganglion a sort of swelling or excrescence, a tumor under the
   skin, Gr. ?: cf. F. ganglion.]
   1. (Anat.)
      (a) A mass or knot of nervous matter, including nerve
          cells, usually forming an enlargement in the course of
          a nerve.
      (b) A node, or gland in the lymphatic system; as, a
          lymphatic ganglion.
          [1913 Webster]

   2. (Med.) A globular, hard, indolent tumor, situated
      somewhere on a tendon, and commonly formed by the effusion
      of a viscid fluid into it; -- called also weeping sinew.
      [1913 Webster]

   Ganglion cell, a nerve cell. See Illust. under Bipolar.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ganglion
    n 1: an encapsulated neural structure consisting of a collection
         of cell bodies or neurons

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "ganglion":
   afferent neuron, autonomic nervous system, axon, brain,
   center of activity, central nervous system, cerebral cortex,
   craniosacral nervous system, dendrite, effector organ,
   efferent neuron, gray matter, internuncial neuron,
   medullary sheath, nerve, nerve center, nerve trunk, nervous system,
   neuron, peripheral nervous system, plexus, sensorium, sensory area,
   sensory cell, solar plexus, spinal cord, synapse,
   thoracolumbar nervous system, vital center, white matter