The Collaborative International Dictionary of English v.0.48:

Mastodon \Mas"to*don\, n. [Gr. masto`s the breast + 'odoy`s,
   'odo`ntos, a tooth. So called from the conical projections
   upon its molar teeth.] (Paleon.)
   An extinct genus of mammals closely allied to the elephant,
   but having less complex molar teeth, and often a pair of
   lower, as well as upper, tusks, which are incisor teeth. The
   species were mostly larger than elephants, and their remains
   occur in nearly all parts of the world in deposits ranging
   from Miocene to late Quaternary time.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mastodon
    n 1: extinct elephant-like mammal that flourished worldwide from
         Miocene through Pleistocene times; differ from mammoths in
         the form of the molar teeth [syn: mastodon, mastodont]