Plaice are a type of flat sea fish. Plaice is this fish eaten as food. a fillet of plaice with sautéed rice and vegetables. plaice a flat sea fish that is eaten. Commercially valuable European flatfish (Pleuronectes platessa). At most 36 in. (90 cm) long, the plaice normally has both eyes on the right side of the head and four to seven bony bumps near its eyes. It is brown with red or orange spots. The American plaice (Hippoglossoides platessoides) is found in both Europe (where it is called the rough dab) and the U.S. It is reddish or brownish and grows to about 24 in. (60 cm) long