The Collaborative International Dictionary of English v.0.48:

Thalamus \Thal"a*mus\, n.; pl. Thalami. [L. thalamus chamber,
   Gr. qa`lamos.]
   1. (Anat.) A mass of nervous matter on either side of the
      third ventricle of the brain; -- called also optic
      thalamus.
      [1913 Webster]

   2. (Bot.)
      (a) Same as Thallus.
      (b) The receptacle of a flower; a torus.
          [1913 Webster]

WordNet (r) 3.0 (2006):

thalamus
    n 1: large egg-shaped structures of grey matter that form the
         dorsal subdivision of the diencephalon