Is the dolphin a fish or a mammal?

It is confusing fact that there is both an air breathing mammal called dolphin that lives in the sea, and also a true fish of the same name. There are two kinds of dolphin fish. One is quite large and grows to about 5 feet in length, and the other is similar in looks but grows to only 2 feet. The favorite food of the dolphin fish is flying fishes and they chase them at speeds of over 35 miles per hour under water.