belonging to the Cetacea, an order of aquatic, chiefly marine mammals, including the whales and dolphins.
a cetacean mammal.

1830–40; < Neo-Latin Cetace(a) name of the order (see cet-, -acea) + -an

cetaceous, adjective
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World English Dictionary
cetacean (sɪˈteɪʃən)
1.  of, relating to, or belonging to the Cetacea, an order of aquatic placental mammals having no hind limbs and a blowhole for breathing: includes toothed whales (dolphins, porpoises, etc) and whalebone whales (rorquals, right whales, etc)
2.  a whale
[C19: from New Latin Cētācea, ultimately from Latin cētus whale, from Greek kētos]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1836, from cetacea (1830), order of marine mammals, Mod.L., from L. cetus, from Gk. ketos "whale."
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
cetacean  [%PREMIUM_LINK%]     (sĭ-tā'shən)  Pronunciation Key 
Any of various, often very large aquatic mammals of the order Cetacea, having a hairless body that resembles that of a fish. Cetaceans have an elongated skull, a flat, horizontal tail, forelimbs modified into broad flippers, and no hind limbs. They breathe through blowholes located usually at the top of the skull. Whales, dolphins, and porpoises are cetaceans. See more at baleen whale, toothed whale.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Though less studied, cetacean matriarchs may shepherd kin to lush feeding
  grounds along migration routes.
The changes in the cetacean ear, head, and brain that have heightened this
  sense are a triumph of adaptation.
In general, large cetacean species emit low frequencies and small cetacean
  species emit high frequencies.
My research focuses on cetacean life history and population structure.
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