This article is from the Dolphin
FAQ, by Jaap van der Toorn
Dolphins are marine mammals, but there is also a fish species that's often called "dolphin" or "dolphin fish". Its scientific name is Coryphaena hippurus. To avoid confusion with the mammal species its Spanish name "dorado" or its Hawaiian name "mahi mahi" is often used.
Because of the confusion between the mammal and the fish species dolphins have in the past erroneously been called porpoises, especially in some US regions, where the fish species is common. In older books you can encounter the name "bottlenose porpoise" for the bottlenose dolphin, for instance. Dolphins and porpoises are however members of different whale families (see 4.5).
You can find more information about the dolphin fish, including its common name in other languages, in the FishBase database, online at http://www.fishbase.org/