The Collaborative International Dictionary of English v.0.48:

Cetacea \Ce*ta"ce*a\, n. pl. [NL., from L. cetus whale, Gr. ?.]
   (Zool.)
   An order of marine mammals, including the whales. Like
   ordinary mammals they breathe by means of lungs, and bring
   forth living young which they suckle for some time. The
   anterior limbs are changed to paddles; the tail flukes are
   horizontal. There are two living suborders:
   (a) The {Mysticete or whalebone whales, having no true
       teeth after birth, but with a series of plates of
       whalebone [see Baleen.] hanging down from the upper jaw
       on each side, thus making a strainer, through which they
       receive the small animals upon which they feed.}
   (b) The {Denticete, including the dolphins and sperm whale,
       which have teeth. Another suborder (Zeuglodontia) is
       extinct. The Sirenia were formerly included in the
       Cetacea, but are now made a separate order.}
       [1913 Webster]

WordNet (r) 3.0 (2006):

Cetacea
    n 1: an order of Eutheria [syn: Cetacea, order Cetacea]