The Collaborative International Dictionary of English v.0.48:

Crawfish \Craw"fish`\ (kr[add]"f[i^]sh`), Crayfish \Cray"fish`\
   (kr[=a]"f[i^]sh`), n.; pl. -fishes or -fish. [Corrupted
   fr. OE. crevis, creves, OF. crevice, F. ['e]crevisse, fr.
   OHG. krebiz crab, G. krebs. See Crab. The ending -fish
   arose from confusion with E. fish.] (Zool.)
   Any decapod crustacean of the family Astacid[ae] (genera
   Cambarus and Cambarus), resembling the lobster, but
   smaller, and found in fresh waters. Crawfishes are esteemed
   very delicate food both in Europe and America. The North
   American species are numerous and mostly belong to the genus
   Cambarus. The blind crawfish of the Mammoth Cave is
   Cambarus pellucidus. The common European species is
   Astacus fluviatilis.

   Syn: crawdad, crawdaddy.
        [1913 Webster]

   2. tiny lobsterlike crustaceans usually boiled briefly.

   Syn: crawdad, ecrevisse.
        [WordNet 1.5]

   3. a large edible marine crustacean having a spiny carapace
      but lacking the large pincers of true lobsters.

   Syn: spiny lobster, langouste, rock lobster, crayfish, sea
        crawfish.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Crayfish \Cray"fish\ (kr[=a]"f[i^]sh), n. (Zool.)
   See Crawfish.
   [1913 Webster]

WordNet (r) 3.0 (2006):

crayfish
    n 1: warm-water lobsters without claws; those from Australia and
         South Africa usually marketed as frozen tails; caught also
         in Florida and California [syn: spiny lobster,
         langouste, rock lobster, crayfish]
    2: tiny lobster-like crustaceans usually boiled briefly [syn:
       crayfish, crawfish, crawdad, ecrevisse]
    3: small freshwater decapod crustacean that resembles a lobster
       [syn: crayfish, crawfish, crawdad, crawdaddy]
    4: large edible marine crustacean having a spiny carapace but
       lacking the large pincers of true lobsters [syn: spiny
       lobster, langouste, rock lobster, crawfish,
       crayfish, sea crawfish]

The Devil's Dictionary (1881-1906):

CRAYFISH, n.  A small crustacean very much resembling the lobster, but
less indigestible.

        In this small fish I take it that human wisdom is admirably
    figured and symbolized; for whereas the crayfish doth move only
    backward, and can have only retrospection, seeing naught but the
    perils already passed, so the wisdom of man doth not enable him to
    avoid the follies that beset his course, but only to apprehend
    their nature afterward.
                                                    Sir James Merivale