Merriam-Webster Collegiate Dictionary - cetacean

 noun  Etymology: ultimately from Latin cetus whale, from Greek ketos  Date: 1835 any of an order (Cetacea) of aquatic mostly marine mammals that includes the whales, dolphins, porpoises, and related forms and that have a torpedo-shaped nearly hairless body, paddle-shaped forelimbs but no hind limbs, one or two nares opening externally at the top of the head, and a horizontally flattened tail used for locomotion  • ~ adjective  • cetaceous adjective