The primary defining characteristic of an adult flounder is its lack of bilateral symmetry, with both of its eyes on one side of its body. Other defining characteristics include protrusible eyes, a dorsal fin that extends onto the head and, in most species, soft fins that lack spines.
"Flounder" is a common name for fish of the taxonomic order Pleuronectiformes, commonly known as "flatfish." All adult flatfish, including flounder, sole, halibuts and others, are bilaterally asymmetrical. When young, flounders are bilaterally symmetrical and swim upright, but early in their development one of the eyes migrates to the other side of the cranium until both eyes are on one side. After this migration, flounder swim on their sides, typically with the "blind" side facing the bottom.
There are several distinct species of flounder; some species swim with their left side facing upward, while others swim with their right side facing upward. Individuals of either orientation exist in other species, such as Platichythys stellatus, commonly called starry flounder.
The bodies of flounder are compressed, or "flat," especially on the "blind" side, while more rounded on the side with eyes. Typically, the pigmentation is notably different between the right and left sides. Adult flounder are typically 5 to 15 inches in length, although some grow up to 3 feet in length.