Whales are the largest species of exclusively aquatic mammals , members of the order Cetacea , which also includes dolphins and porpoises . They include the largest known animals ever evolved.

The term whale is ambiguous: it can refer to all cetaceans, to just the larger ones, or only to members of particular families within the order Cetacea. The latter definition is the one followed here. Whales are those cetaceans which are neither dolphins (i.e. members of the families Delphinidae or Platanistoidea ), nor porpoises . This can lead to some confusion because Orcas ("Killer Whales") and Pilot Whales have "whale" in their name, but they are dolphins for the purpose of classification.