a very large animal that lives in the sea and looks like a fish, but is actually a mammal.——朗文当代大辞典

Any of the larger fish-like marine mammals of the order Cetacea, which have fore-limbs like fins and a tail with horizontal flukes, and are hunted for their oil and whalebone; in wider (scientific) use, any cetacean of the groups Mystacoceti or whalebone-whales, and Odontoceti or toothed whales (which are distinguished by the names dolphin, grampus, porpoise, etc.).——牛津当代大辞典  @en