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| Long-finned pilot whale |
The term "dolphin" is not, strictly speaking, a scientific one. It refers, in common parlance, to any small cetacean, often even including porpoises, which are a different (if related) kind of animal. Even ignoring the porpoises, however, not all dolphins are members of the dolphin family, technically referred to as "delphinids". This is because some freshwater animals are not closely related to the dolphins proper (or, indeed, to the porpoises). We call them "dolphins" because they're about the right size, a similar shape, and... well, we don't have a better word, at least for them all collectively.
But it works the other way, too. Not all members of the dolphin family are commonly called "dolphins". With the exception of the
melon-headed whale, which it's hard to think of as anything other than a dolphin, this is because they're too big. We call them "whales" - another term that doesn't map to anything scientifically - since that's what we call any large cetacean.
