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}.