The Collaborative International Dictionary of English v.0.48:

Axolotl \Ax"o*lotl\, n. [The native name.] (Zool.)
   An amphibian of the salamander tribe found in the elevated
   lakes of Mexico; the siredon.
   [1913 Webster]

   Note: When it breeds in captivity the young develop into true
         salamanders of the genus Amblystoma. This also occurs
         naturally under favorable conditions, in its native
         localities; although it commonly lives and breeds in a
         larval state, with persistent external gills. See
         Siredon.
         [1913 Webster]

WordNet (r) 3.0 (2006):

axolotl
    n 1: larval salamander of mountain lakes of Mexico that usually
         lives without metamorphosing [syn: axolotl, mud puppy,
         Ambystoma mexicanum]