Whale, any of the marine mammals in the order Cetacea. They are unique among all mammals in that they carry out their complete life history, from birth to death, in water. The term cetacean is used to embrace all known species (about 79) of whales, dolphins, and porpoises.

Evidence indicates that whales descended from a four-legged land animal, perhaps a primitive ungulate (hoofed mammal) that may also have given rise to modern ungulates. The earliest known whale fossils are 52 million years old, but many scientists estimate that whales date from 60 million years ago. Fossilized cetacean skeletons dating back to the Eocene epoch (56.5 million to 35.4 million years ago) were recently discovered in Pakistan. These fossils indicate that early whales swam by undulating their vertebral column, thus forcing their feet up and down in a way similar to modern otters.

Whales are divided into two suborders: toothed whales and baleen whales. Most smaller whales, and all the dolphins and porpoises, belong to the toothed whale suborder. Those more than 4 to 5 m (13 to 16 ft) long are generally referred to as whales, whereas smaller species are known as dolphins or porpoises. Toothed whales have teeth that are uniform in size and shape although they vary considerably in number, and they feed on fish and invertebrates such as squid and crustaceans; one species, the killer whale, has a more varied diet that includes seabirds and marine mammals. A few species are commercially valuable as exhibits in aquariums and oceanariums, and some of the smaller whales are hunted to a limited extent. One toothed whale, the sperm whale, sometimes known as a cachalot, is quite large: the male grows to a length of 18.3 m (60 ft), and the female grows to a length of 12.2 m (40 ft). It was heavily hunted in the past.

The rest of the larger whales belong to the baleen whale suborder. In this group of ten species—all of which have been or are currently being hunted—teeth have been replaced with large structures, known as baleen plates, that hang like vertical Venetian blinds from the upper jaw. The plates number 160 to 395 on each side, are frayed into bristles on their inner edges, and are used to capture the plankton or krill on which the animals subsist. When feeding, a baleen whale swims with its mouth open in order to engulf plankton and seawater by the ton.