The Collaborative International Dictionary of English v.0.48:

balloonfish \bal*loon"fish`\, balloon fish \bal*loon" fish`\
   (Zool.)
   A fish of the genus Diodon (such as Diodon holocanthus)
   or the genus Tetraodon, having the power of distending its
   body by taking air or water into its dilatable esophagus. It
   is similar to but smaller than the porcupinefish. See
   Globefish, and Bur fish.
   [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

balloonfish
    n 1: similar to but smaller than porcupinefish [syn:
         balloonfish, Diodon holocanthus]