NOUN (2)

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;

The Collaborative International Dictionary of English v.0.48:

Flatfish \Flat"fish`\, n. (Zool.)
   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.
   [1913 Webster] flat foot

WordNet (r) 3.0 (2006):

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