The order Cetacea (IPAEng: sɪˈteɪʃiə, L. cetus, whale) includes whales, dolphins and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal", was more general. It comes from Ancient Greek (lang: kētos), meaning "whale" or "any huge fish or sea monster". In Greek mythology the monster Perseus defeated was called Ceto, which is depicted by the constellation of Cetus. Cetology is the branch of marine science associated with the study of cetaceans.

Cetaceans are the mammals most fully adapted to aquatic life. Their body is fusiform (spindle-shaped). The forelimbs are modified into flippers. The tiny hindlimbs are vestigial; they do not attach to the backbone and are hidden within the body. The tail has horizontal flukes. Cetaceans are nearly hairless, and are insulated by a thick layer of blubber. As a group, they are noted for their high intelligence.

The order Cetacea contains about ninety species, all marine except for four species of freshwater dolphins. The order is divided into two suborders, Mysticeti (baleen whales) and Odontoceti (toothed whales, which includes dolphins and porpoises). The species range in size from the Commerson's Dolphin and Tucuxi to the Blue Whale, the world's largest ever animal.

Respiration, vision, hearing and echolocation

As mammals, cetaceans need to breathe air. Because of this, they need to come to the water's surface to exhale carbon dioxide and inhale a fresh supply of oxygen. During diving, a muscular action closes the blowholes (nostrils), which remain closed until the cetacean next breaks the surface; when it surfaces, the muscles open the blowholes and warm air is exhaled.

Cetaceans' blowholes have evolved to a position on top of the head, allowing more time to expel stale air and inhale fresh air. When the stale air, warmed from the lungs, is exhaled, it condenses as it meets the cold air outside. As with a terrestrial mammal breathing out on a cold day, a small cloud of 'steam' appears. This is called the 'blow' or 'spout' and is different in terms of shape, angle and height, for each cetacean species. Cetaceans can be identified at a distance, using this characteristic, by experienced whalers or whale-watchers.

Cetaceans can go underwater for much longer periods of time than other mammals. Their duration under water varies greatly between species due to large physiological differences between many members of this Order. There are two studiedFact: date=April 2008 advantages of cetacean physiology that let this Order (and other marine mammals) forage underwater for extended periods of time without breathing at the water surface.