Is a killer whale really a whale or a dolphin?

It is a mammal, a member of the dolphin family.

The killer whale's scientific name is 'Orcinus Orca', And it is actually the largest of the dolphin family. It got its name Killer Whale, which was shortened from Killer OF whales, as they sometimes kill other dolphins, and whales. They are NOT a porpoise, whale or fish (although the name porpoise is sometimes applied to them).