The Collaborative International Dictionary of English v.0.48:

Prawn \Prawn\, n. [OE. prane, of unknown origin; cf. L. perna a
   sea mussel.] (Zool.)
   Any one of numerous species of large shrimplike Crustacea
   having slender legs and long antenn[ae]. They mostly belong
   to the genera Pandalus, Pal[ae]mon, Pal[ae]monetes, and
   Peneus, and are much used as food. The common English prawn
   is Pal[ae]mon serratus.
   [1913 Webster]

   Note: The name is often applied to any large shrimp.
         [1913 Webster]

WordNet (r) 3.0 (2006):

prawn
    n 1: any of various edible decapod crustaceans [syn: prawn,
         shrimp]
    2: shrimp-like decapod crustacean having two pairs of pincers;
       most are edible
    v 1: fish for prawns

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "prawn":
   Dungeness crab, Japanese crab, blue point, clam, coquillage, crab,
   crawdad, crawfish, crayfish, langouste, limpet, littleneck clam,
   lobster, mussel, oyster, periwinkle, quahog, scallop, shellfish,
   shrimp, snail, soft-shell crab, steamer, whelk