Source : Webster's Revised Unabridged Dictionary (1913)

Flatfish \Flat"fish`\, n. (Zo["o]l.)
   Any fish of the family {Pleuronectid[ae]}; esp., the winter
   flounder ({Pleuronectes Americanus}). The flatfishes have the
   body flattened, swim on the side, and have eyes on one side,
   as the flounder, turbot, and halibut. See {Flounder}.

Source : WordNet®

flatfish
     n 1: sweet lean whitish flesh of any of numerous thin-bodied
          fish; usually served as thin fillets
     2: any of several families of fishes having flattened bodies
        that swim along the sea floor on one side of the body with
        both eyes on the upper side
     [also: {flatfishes} (pl)]