Halibut, a flatfish related to the flounder. The name is taken from the Middle English words haly, meaning “holy,” and butte, meaning “flatfish,” because halibut was traditionally eaten on holy days. The halibut is longer, thicker, and heavier than other flatfish. It is dark brown on the upper side, white beneath. The eyes are on the upper side. The male halibut rarely exceeds 40 pounds (18 kg). Some females reach a length of 9 feet (2.7 m) and weigh about 700 pounds (320 kg).

A 20-year-old female may lay 2,500,000 eggs in a season, usually at a depth of 900 feet (270 m). Here the eggs drift with the current until the young are hatched. A young halibut swims upright as do most other fish. It has an eye on either side of its head. As the fish develops, it begins to swim on its left side, and the left eye gradually shifts to the right (top) side. Halibut mature at about 12 years and may live 40 years. Much of the fish's life is spent feeding on the ocean bottom, sometimes 3,000 feet (900 m) below the surface.

The halibut is a valuable food fish but is declining in importance, particularly in the Atlantic. The oil of halibut liver is much richer in vitamins A and D than is cod-liver oil.

Pacific halibut are caught off western North America from Alaska to Puget Sound, and off Hokkaido, Japan. Atlantic halibut are found off eastern Canada, and near Greenland, Iceland, and northern Europe.

Pacific halibut are usually caught by hook-and-line, Atlantic halibut by hook-and-line and by otter trawls. (Otter trawls are funnel-shaped seines [nets] that fishing boats tow along the ocean bottom.) Most halibut weigh 40 to 100 pounds (18 to 45 kg) when marketed.

The Pacific halibut is Hippoglossus stenolepis; Atlantic, H. hippoglossus. Halibut belong to the family Hippoglossoides.