Source : Webster's Revised Unabridged Dictionary (1913)

Claustrum \Claus"trum\, n.; pl. {Claustra}. [L., a bolt or bar.]
   (Anat.)
   A thin lamina of gray matter in each cerebral hemisphere of
   the brain of man. -- {Claus"tral}, a.

Source : WordNet®

claustrum
     n : a layer of gray matter in the brain adjacent to the
         lenticular nucleus