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: Webster's Revised Unabridged Dictionary (1913)