Dolphins are marine mammals closely related to whales and porpoises. There are almost forty species of dolphin in 17 genera. They vary in size from 1.2 m (4 ft) and 40 kg (90 lb) (Maui's dolphin), up to 9.5 m (30 ft) and 10 tonnes (9.8 long tons; 11 short tons) (the orca or killer whale). They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, eating mostly fish and squid.
The family Delphinidae is the largest in the Cetacean order, and evolved relatively recently, about ten million years ago, during the Miocene. Dolphins are among the most intelligent animals, and their often friendly appearance, an artifact of the "smile" of their mouthline, and seemingly playful attitude have made them very popular in human culture.
The name is originally from Greek δελφίς (delphís), "dolphin", which was related to the Greek δελφύς (delphus), "womb". The animal's name can therefore be interpreted as meaning "a 'fish' with a womb". The name was transmitted via the Latin delphinus (the romanization of the later Greek δελφῖνος – delphinos), which in Medieval Latin became dolfinus and in Old French daulphin, which reintroduced the ph into the word. The term mereswine (that is, "sea pig") has also historically been used.