Cetacea is an infraorder of aquatic mammals belonging to the order Artiodactyla that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel themselves through the water with powerful up-and-down movement of their tail which ends in a paddle-like fluke, using their flipper-shaped forelimbs to maneuver.

Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins and porpoises. Dolphins and porpoises may be considered whales from a formal, cladistic perspective. Whales, dolphins and porpoises belong to the order Cetartiodactyla, which consists of even-toed ungulates. Their closest non-cetacean living relatives are the hippopotamuses, from which they and other cetaceans diverged about 54 million years ago. The two parvorders of whales, baleen whales (Mysticeti) and toothed whales (Odontoceti), are thought to have had their last common ancestor around 34 million years ago. Mysticetes include four extant (living) families: Balaenopteridae, Balaenidae, Cetotheriidae, and Eschrichtiidae. Odontocetes include the Monodontidae, Physeteridae, Kogiidae, and Ziphiidae, as well as the six families of dolphins and porpoises which are not considered whales in the informal sense.