Although dolphins, porpoises and whales (blue, beluga, humpback, killer, sperm, etc) look like fish, technically they are classified as mammals. As with all mammals, dolphins/whales bear their young live and the young feed from the mother's mammaries.
According to Merriam-Webster Dictionar a fish is
any of numerous cold-blooded strictly aquatic craniate vertebrates that include the bony fishes and usually the cartilaginous and jawless fishes and that have typically an elongated somewhat spindle-shaped body terminating in a broad caudal fin, limbs in the form of fins when present at all, and a 2-chambered heart by which blood is sent through thoracic gills to be oxygenated
Dolphins are warm blooded (constant body temperature instead of temperature of the environment), have a 4-chambered heart and do not have gills by which to extract dissolved oxygen from the water. They have a blow hole through which they breathe air from the atmosphere.
Dolphins are considered to be highly intelligent animals. However, there is actually a dolphin
fish, (also posted by Br. Woodrow) but it looks quite different. The dolphin
fish is also called a mahi mahi.