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.
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
Opossum mouse. (Zool.) See Flying mouse, under Flying.
Opossum shrimp (Zool.), any schizopod crustacean of the
genus Mysis and allied genera. See Schizopoda.