Though it swims in the ocean, a dolphin is not a fish. Instead, dolphins are marine mammals, meaning they have all of the physical characteristics that require a mammalian classification but live in the oceans.
Dolphins are among a family of very intelligent marine mammals that includes orcas and pilot whales. In order to be classified as a mammal, an animal must have some features that distinguish it from the other animal groups, such as reptiles and birds. Mammals have hair and mammary glands that excrete milk to feed offspring. Mammals are also distinguished by their three middle ear bones, and their developed region of the brain known as the neocortex.