The Collaborative International Dictionary of English v.0.48:

Cetacean \Ce*ta"cean\, n. (Zool.)
   One of the Cetacea.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cetacean
    adj 1: of or relating to whales and dolphins etc [syn:
           cetacean, cetaceous]
    n 1: large aquatic carnivorous mammal with fin-like forelimbs no
         hind limbs, including: whales; dolphins; porpoises;
         narwhals [syn: cetacean, cetacean mammal, blower]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "cetacean":
   Loch Ness monster, alevin, baleen whale, beluga, benthon, benthos,
   blackfish, blue whale, cachalot, dolphin, finback, fingerling,
   fish, fry, game fish, grilse, humpback, killer, killer whale,
   kipper, man-eater, man-eating shark, marine animal, minnow, minny,
   narwhal, nekton, panfish, plankton, porpoise, salmon, sea monster,
   sea pig, sea serpent, sea snake, shark, smolt, sperm whale, sponge,
   sulfur-bottom whale, tropical fish, whale