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]

Moby Thesaurus II by Grady Ward, 1.0:

268 Moby Thesaurus words for "opossum":
   Angora goat, Arctic fox, Belgian hare, Caffre cat, Cape polecat,
   Indian buffalo, Kodiak bear, Virginia deer, aardvark, aardwolf,
   alpaca, anteater, antelope, antelope chipmunk, aoudad, apar, ape,
   armadillo, ass, aurochs, badger, bandicoot, bar, bassarisk, bat,
   bear, beaver, bettong, binturong, bison, black bear, black buck,
   black cat, black fox, black sheep, blue fox, bobcat, brown bear,
   brush deer, brush wolf, buffalo, buffalo wolf, burro, burro deer,
   cachalot, camel, camelopard, capybara, carabao, caribou, carpincho,
   cat, cat-a-mountain, catamount, cattalo, cavy, chamois, cheetah,
   chevrotain, chimp, chimpanzee, chinchilla, chipmunk, cinnamon bear,
   coon, coon cat, cotton mouse, cotton rat, cougar, cow, coyote,
   coypu, deer, deer tiger, dingo, dog, donkey, dormouse, dromedary,
   echidna, eland, elephant, elk, ermine, eyra, fallow deer, ferret,
   field mouse, fisher, fitch, flying phalanger, foumart, fox,
   fox squirrel, gazelle, gemsbok, genet, giraffe, glutton, gnu,
   gnu goat, goat, goat antelope, gopher, grizzly bear,
   ground squirrel, groundhog, guanaco, guinea pig, hamster, hare,
   harnessed antelope, hartebeest, hedgehog, hippopotamus, hog, horse,
   hyena, hyrax, ibex, jackal, jackass, jackrabbit, jaguar,
   jaguarundi, jerboa, jerboa kangaroo, kaama, kangaroo,
   kangaroo mouse, kangaroo rat, karakul, kinkajou, kit fox, koala,
   lapin, lemming, leopard, leopard cat, lion, llama, lynx, mammoth,
   mara, marmot, marten, mastodon, meerkat, mink, mole, mongoose,
   monk, monkey, moose, mouflon, mountain goat, mountain lion,
   mountain sheep, mouse, mousehound, mule, mule deer, muntjac,
   musk deer, musk hog, musk-ox, muskrat, musquash, nilgai, nutria,
   ocelot, okapi, onager, oont, otter, ounce, ox, pack rat, painter,
   panda, pangolin, panther, peccary, peludo, phalanger, pig,
   pine mouse, platypus, pocket gopher, pocket mouse, pocket rat,
   polar bear, polar fox, polecat, porcupine, possum, pouched rat,
   poyou, prairie dog, prairie wolf, pronghorn, puma, quill pig,
   rabbit, raccoon, rat, red deer, red squirrel, reindeer, rhinoceros,
   roe, roe deer, roebuck, sable, serval, sheep, shrew, shrew mole,
   sika, silver fox, skunk, sloth, snowshoe rabbit, springbok,
   squirrel, stoat, suslik, swamp rabbit, swine, takin, tamandua,
   tamarin, tapir, tarpan, tatou, tatou peba, tatouay, tiger,
   tiger cat, timber wolf, tree shrew, urus, vole, wallaby, warthog,
   water buffalo, waterbuck, weasel, wharf rat, whistle-pig, whistler,
   white fox, wild ass, wild boar, wild goat, wild ox, wildcat,
   wildebeest, wolf, wolverine, wombat, wood rat, woodchuck,
   woolly mammoth, yak, zebra, zebu, zoril