The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

claustrum
    n 1: a layer of grey matter in the brain adjacent to the
         lenticular nucleus