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