The Collaborative International Dictionary of English v.0.48:

Tetrodon \Tet"ro*don\, n. [Tetra- + Gr. ?, ?, tooth.] (Zool.)
   Any one of numerous species of plectognath fishes belonging
   to Tetrodon and allied genera. Each jaw is furnished with
   two large, thick, beaklike, bony teeth. [Written also
   tetradon.]
   [1913 Webster]

   Note: The skin is usually spinous, and the belly is capable
         of being greatly distended by air or water. It includes
         the swellfish, puffer
   (a), and similar species.
       [1913 Webster]