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]