The Collaborative International Dictionary of English v.0.48:

Pallium \Pal"li*um\, n.; pl. L. Pallia, E. Palliums. [L. See
   Pall the garment.]
   1. (Anc. Costume) A large, square, woolen cloak which
      enveloped the whole person, worn by the Greeks and by
      certain Romans. It is the Roman name of a Greek garment.
      [1913 Webster]

   2. (R. C. Ch.) A band of white wool, worn on the shoulders,
      with four purple crosses worked on it; a pall.
      [1913 Webster]

   Note: The wool is obtained from two lambs brought to the
         basilica of St. Agnes, Rome, and blessed. It is worn by
         the pope, and sent to patriarchs, primates, and
         archbishops, as a sign that they share in the plenitude
         of the episcopal office. Before it is sent, the pallium
         is laid on the tomb of St. Peter, where it remains all
         night.
         [1913 Webster]

   3. (Zool.)
      (a) The mantle of a bivalve. See Mantle.
      (b) The mantle of a bird.
          [1913 Webster]

WordNet (r) 3.0 (2006):

pallium
    n 1: the layer of unmyelinated neurons (the grey matter) forming
         the cortex of the cerebrum [syn: cerebral cortex,
         cerebral mantle, pallium, cortex]
    2: (zoology) a protective layer of epidermis in mollusks or
       brachiopods that secretes a substance forming the shell [syn:
       mantle, pallium]
    3: (Roman Catholic Church) vestment consisting of a band
       encircling the shoulders with two lappets hanging in front
       and back
    4: cloak or mantle worn by men in ancient Rome