The Collaborative International Dictionary of English v.0.48:

Flying squirrel \Fly"ing squir"rel\ (? or ?). (Zool.)
   One of a group of squirrels, of the genus Glaucomys
   (formerly Pteromus and Sciuropterus [1913 Webster]),
   especially Glaucomys volans and Glaucomys sabrinus,
   having parachute-like folds of skin extending from the fore
   to the hind legs, which enable them to make very long,
   gliding leaps.
   [1913 Webster +PJC]

   Note: The species of Pteromys are large, with bushy tails,
         and inhabit southern Asia and the East Indies; those of
         Sciuropterus are smaller, with flat tails, and inhabit
         the northern parts of Europe, Asia, and America. The
         American species (Sciuropterus volucella) is also
         called Assapan. The Australian flying squirrels, or
         flying phalangers, are marsupials. See Flying
         phalanger (above).
         [1913 Webster]

WordNet (r) 3.0 (2006):

Glaucomys volans
    n 1: small large-eyed nocturnal flying squirrel of eastern
         United States [syn: southern flying squirrel, Glaucomys
         volans]