The name is originally from Greek δελφίς (delphís), "dolphin",^[1] which was related to the Greek δελφύς (delphus), "womb".^[2] The animal's name can therefore be interpreted as meaning "a 'fish' with a womb".^[3] The name was transmitted via the Latin delphinus^[4] (the romanization of the later Greek δελφῖνος – delphinos^[5]), 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.^[6]
The word is used in a few different ways. It can mean:

• any member of the family Delphinidae (oceanic dolphins),
• any member of the family Delphinidae or the superfamily Platanistoidea (oceanic and river dolphins),
• any member of the suborder Odontoceti (toothed whales; these include the above families and some others),
• and is used casually as a synonym for bottlenose dolphin, the most common and familiar species of dolphin.

This article uses the second definition and does not describe porpoises (suborder Odontoceti, family Phocoenidae). Orcas and some closely related species belong to the Delphinidae family and therefore qualify as dolphins, even though they are called whales in common language.
A group of dolphins is called a "school" or a "pod". Male dolphins are called "bulls", females "cows" and young dolphins are called "calves".^[7]