The Collaborative International Dictionary of English v.0.48:

Opossum \O*pos"sum\, n. [Of N. American Indian origin.] (Zool.)
   Any American marsupial of the genera Didelphys and
   Chironectes; called also possum. The common species of
   the United States is Didelphys Virginiana.
   [1913 Webster]

   Note: Several related species are found in South America. The
         water opossum of Brazil (Chironectes variegatus),
         which has the hind feet, webbed, is provided with a
         marsupial pouch and with cheek pouches. It is called
         also yapock.
         [1913 Webster]

   Opossum mouse. (Zool.) See Flying mouse, under Flying.


   Opossum shrimp (Zool.), any schizopod crustacean of the
      genus Mysis and allied genera. See Schizopoda.
      [1913 Webster]

WordNet (r) 3.0 (2006):

opossum
    n 1: small furry Australian arboreal marsupials having long
         usually prehensile tails [syn: phalanger, opossum,
         possum]
    2: nocturnal arboreal marsupial having a naked prehensile tail
       found from southern North America to northern South America
       [syn: opossum, possum]