The name "killer whale" derives from the Spanish "ballena asesina" ("whale killer" in English), evidently coming from sailors who observed them hunting whales. English-speaking scientists most often use the term "killer whale."
"Killer whale" advocates point out that its naming heritage is not limited to Spanish sailors. Indeed, the genus name Orcinus means "of or belonging to the kingdom of the dead", and although the name Orca (in use since antiquity) is probably not etymologically related, the assonance might have given some people the idea that it meant "whale that brings death" or "demon from hell." The name is also similar to Orcus, a Roman god of the underworld.
Ancient Romans originally applied Orca (plural Orcas) to these animals, possibly borrowing it from the Greek ὄρυξ, which referred (among other things) to a whale species. Since the 1960s, Orca has steadily grown in popularity; both names are now used. The term "orca" is preferred by some to avoid the negative connotations of "killer", and because the species is more closely related to dolphins than to other whales.
They are sometimes referred to as blackfish, a name also used for other whale species. Grampus is a former name for the species, but is now seldom used. This meaning of grampus should not be confused with the Grampus genus, whose only member is Risso's Dolphin.