The Collaborative International Dictionary of English v.0.48:

Porpoise \Por"poise\, n. [OE. porpeys, OF. porpeis, literally,
   hog fish, from L. porcus swine + piscis fish. See Pork, and
   Fish.]
   1. (Zool.) Any small cetacean of the genus Phoc[ae]na,
      especially Phoc[ae]na communis, or Phoc[ae]na
      phoc[ae]na, of Europe, and the closely allied American
      species (Phoc[ae]na Americana). The color is dusky or
      blackish above, paler beneath. They are closely allied to
      the dolphins, but have a shorter snout. Called also
      harbor porpoise, herring hag, puffing pig, and
      snuffer.
      [1913 Webster]

   2. (Zool.) A true dolphin (Delphinus); -- often so called
      by sailors.
      [1913 Webster]

   Skunk porpoise, or Bay porpoise (Zool.), a North American
      porpoise (Lagenorhynchus acutus), larger than the common
      species, and with broad stripes of white and yellow on the
      sides. See Illustration in Appendix.
      [1913 Webster]

WordNet (r) 3.0 (2006):

porpoise
    n 1: any of several small gregarious cetacean mammals having a
         blunt snout and many teeth

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "porpoise":
   Loch Ness monster, alevin, benthon, benthos, cetacean, dolphin,
   fingerling, fish, fry, game fish, grilse, kipper, man-eater,
   man-eating shark, marine animal, minnow, minny, nekton, panfish,
   plankton, salmon, sea monster, sea pig, sea serpent, sea snake,
   shark, smolt, sponge, tropical fish, whale