The Collaborative International Dictionary of English v.0.48:

Vole \Vole\, n. (Zool.)
   Any one of numerous species of micelike rodents belonging to
   Arvicola and allied genera of the subfamily Arvicolinae.
   They have a thick head, short ears, and a short hairy tail.
   [1913 Webster]

   Note: The water vole, or water rat, of Europe (Arvicola
         amphibius) is a common large aquatic species. The
         short-tailed field vole (Arvicola agrestis) of
         Northern and Central Europe, and Asia, the Southern
         field vole (Arvicola arvalis), and the Siberian root
         vole (Arvicola oeconomus), are important European
         species. The common species of the Eastern United
         States (Arvicola riparius) (called also meadow
         mouse) and the prairie mouse (Arvicola austerus) are
         abundant, and often injurious to vegetation. Other
         species are found in Canada.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Arvicola amphibius
    n 1: common large Eurasian vole [syn: water vole, water rat,
         Arvicola amphibius]