a large North American ranid frog (Rana catesbeiana) that has a deep, loud croak
See bullfrog in American Heritage Dictionary 4
(bo͝olˈfrôgˌ, -frŏgˌ)
noun
Any of several large, heavy-bodied frogs, chiefly of the genus Rana and especially R. catesbeiana, native to North America and having a characteristic deep resonant croak.