The Collaborative International Dictionary of English v.0.48:

Cottontail \Cot"ton*tail`\ (k[o^]t"t'n*t[=a]l`), n. (Zool.)
   The American wood rabbit (Lepus sylvaticus); -- also called
   Molly cottontail.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cottontail
    n 1: common small rabbit of North America having greyish or
         brownish fur and a tail with a white underside; a host for
         Ixodes pacificus and Ixodes scapularis (Lyme disease ticks)
         [syn: wood rabbit, cottontail, cottontail rabbit]