Source : Webster's Revised Unabridged Dictionary (1913)

Bullfrog \Bull"frog`\, n. (Zo["o]l.)
   A very large species of frog ({Rana Catesbiana}), found in
   North America; -- so named from its loud bellowing in spring.

Source : WordNet®

bullfrog
     n : largest North American frog; highly aquatic with a
         deep-pitched voice [syn: {Rana catesbeiana}]