Source : Webster's Revised Unabridged Dictionary (1913)

Mastodon \Mas"to*don\, n. [Gr. ? the breast + ?, ?, 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 romains
   occur in nearly all parts of the world in deposits ranging
   from Miocene to late Quaternary time.

Source : WordNet®

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