Halibut is a flatfish, genus Hippoglossus, from the family of the right-eye flounders (Pleuronectidae). Other flatfish are also called halibut. The name is derived from haly (holy) and butt (flat fish), for its popularity on Catholic holy days. Halibut are demersal fish which live in the North Pacific and the North Atlantic oceans. They are highly-regarded food fish.

The halibut is the largest flat fish, averaging 11–13.5 kilograms (24–30 lb), but catch as large as 333 kilograms (730 lb) are reported; the largest recently recorded was 245 kilograms (540 lb) taken off the coast of Northern Norway and 2.5 metres (8.2 ft) long. They are gray-black on the top side with an off-white underbelly and have very small scales invisible to the naked eye embedded in their skin. At birth they have an eye on each side of the head, and swim like a salmon. After six months one eye migrates to the other side, making them look more like flounder. At the same time the stationary-eyed side darkens to match the top side, while the other side remains white. This color scheme disguises halibut from above (blending with the ocean floor) and from below (blending into the light from the sky) and is known as countershading.