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]

The Collaborative International Dictionary of English v.0.48:

Possum \Pos"sum\, n. [Shortened from opossum.] (Zool.)
   An opossum. [Colloq. U. S.]
   [1913 Webster]

   To play possum, To act possum, to feign ignorance,
      indifference or inattention, with the intent to deceive;
      to dissemble; -- in allusion to the habit of the opossum,
      which feigns death when attacked or alarmed.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "possum":
   Cape polecat, ape, bar, bear, cavy, chimp, chimpanzee, coon,
   ferret, foumart, glutton, groundhog, guinea pig, hedgehog, monk,
   monkey, mousehound, opossum, polecat, porcupine, prairie dog,
   quill pig, raccoon, skunk, weasel, whistle-pig, wolverine,
   woodchuck, zoril