Halibut is the common name for three flatfish in the genera Hippoglossus and Reinhardtius from the family of right-eye flounders and, in some regions, and less commonly, other species of large flatfish.

A flatfish is a member of the ray-finned demersal fish order Pleuronectiformes, also called the Heterosomata, sometimes classified as a suborder of Perciformes. In many species, both eyes lie on one side of the head, one or the other migrating through or around the head during development. Some species face their left sides upward, some face their right sides upward, and others face either side upward.