The term ‘dolphin’ can be used to refer to, under the suborder Odontoceti, all the species in the family Delphinidae (marine dolphins including orcas and pilot whales) and the river dolphin families Iniidae (South American river dolphins), Pontoporiidae (La Plata dolphin), Lipotidae (Yangtze river dolphin) and Platanistidae (Ganges river dolphin and Indus river dolphin).

This term has often been misused in the US, mainly in the fishing industry, where all small cetaceans (dolphins and porpoises) are considered porpoises, while the fish dorado is called dolphin fish. In common usage the term ‘whale’ is used only for the larger cetacean species,[10] while the smaller ones with a beaked or longer nose are considered ‘dolphins’.

The name ‘dolphin’ is used casually as a synonym for bottlenose dolphin, the most common and familiar species of dolphin. Killer whales also belong to the family Delphinidae and therefore qualify as dolphins. Though the terms ‘dolphin’ and ‘porpoise’ are sometimes used interchangeably, porpoises are not considered dolphins and have different physical features such as a shorter beak and spade-shaped teeth; they also differ in their behavior. Porpoises belong to the family Phocoenidae and share a common ancestry with the Delphinidae.

A group of dolphins is called a “school” or a “pod”. Male dolphins are called “bulls”, females “cows” and young dolphins are called “calves”.