- any fish of the order Heterosomata (Pleuronectiformes), including the halibut, sole, flounder, etc., having a greatly compressed body and swimming on one side, with both eyes on the upper side in the adult.
- any marine spiny-finned fish of the order Heterosomata, including the halibut, plaice, turbot, and sole, all of which (when adult) swim along the sea floor on one side of the body, which is highly compressed and has both eyes on the uppermost side
