 The term " whale " is sometimes used interchangeably with dolphins and porpoises , acting as a synonym for Cetacea . Six species of dolphins have the word " whale " in their name , collectively known as blackfish : the killer whale , the melon @-@ headed whale , the pygmy killer whale , the false killer whale , and the two species of pilot whales , all of which are classified under the family Delphinidae ( oceanic dolphins ) . Each species has a different reason for it , for example , the killer whale was named " Ballena asesina " by Spanish sailors , which translates directly to " whale assassin " or " whale killer " , but is more often translated to " killer whale " . 
