Source : Webster's Revised Unabridged Dictionary (1913)

Crayfish \Cray"fish\ (kr[=a]"f[i^]sh), n. (Zo["o]l.)
   See {Crawfish}.

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.] (Zo["o]l.)
   Any crustacean of the family {Astacid[ae]}, 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}.

Source : WordNet®

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}]
     2: tiny lobster-like crustaceans usually boiled briefly [syn: {crawfish},
         {crawdad}, {ecrevisse}]
     3: small freshwater decapod crustacean that resembles a lobster
        [syn: {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}, {sea
        crawfish}]
     [also: {crayfishes} (pl)]