Cod is the common name for the genus Gadus of fish, belonging to the family Gadidae, and is also used in the common name of a variety of other fishes. Cod is a popular food fish with a mild flavor, low fat content, and a dense white flesh that flakes easily. Cod livers are processed to make cod liver oil, an important source of Vitamin A, Vitamin D and omega-3 fatty acids (EPA and DHA).

In the United Kingdom, Atlantic cod, despite now being an endangered species,

is still one of the most common kinds of fish to be found in fish and chips, along with haddock and plaice.

It is also well known for being largely consumed in Portugal, where it is considered a treasure of the nation's cuisine.

Bacalhau
Bacalhau means codfish in Portuguese, but the word almost always refers to the dry, salted codfish product called clipfish, as fresh cod is rarely consumed in Portugal. The word bacalhau is however also used when referring to dishes with clipfish.

Species in genus Gadus
At various times in the past, a very considerable number of species have been classified in this genus. However the great majority of them are now either classified in other genera, or have been recognised as simply forms of one of three species. Modern taxonomy, therefore, recognises only three species in this genus:

• Atlantic cod, Gadus morhua
• Pacific cod, Gadus macrocephalus
• Greenland cod, Gadus ogac

All these species have a profusion of common names, most of them including the word "cod". Many common names have been used of more than one species, in different places or at different times.

Related species called cod
Cod forms part of the common name of many other fish no longer classified in the genus Gadus. Many of these are members of the family Gadidae, and several were formerly classified in genus Gadus; others are members of three related families whose names include the word "cod": the morid cods, Moridae (100 or so species); the eel cods, Muraenolepididae (4 species); and the Eucla cod, Euclichthyidae (1 species). The tadpole cod family (Ranicipitidae) has now been absorbed within Gadidae.

Species within the order Gadiformes that are commonly called cod include:

• Arctic cod Arctogadus glacialis
• East Siberian cod Arctogadus borisovi
• Saffron cod Eleginus gracilis
• Polar cod Boreogadus saida
• Rock cod Lotella rhacina
• Poor cod Trisopterus minutus
• Pelagic cod Melanonus gracilis
• Small-headed cod Lepidion microcephalus
• Tadpole cod Guttigadus globosus
• Eucla cod Euclichthys polynemus
• Some other related fish have common names derived from "cod", such as codling, codlet or tomcod. ("Codling" is also used as a name for a young cod.)

Unrelated species called cod
However there are also fish commonly known as cod that are quite unrelated to the genus Gadus. Part of this confusion of names is market-driven. Since the decline in cod stocks has made the Atlantic cod harder to catch, cod replacements are marketed under names of the form "x cod", and culinary rather than phyletic similarity has governed the emergence of these names. A very large number of fish have thus been named as some kind of cod at some time. The following species, however, seem to have well established common names including the word "cod"; note that all are Southern Hemisphere species.

Order Perciformes:

• Norwegian fisher with codMurray cod Maccullochella peelii peelii
• Eastern freshwater cod Maccullochella ikei
• Mary River cod Maccullochella peelii mariensis
• Trout cod Maccullochella macquariensis
• Sleepy cod Oxyeleotris lineolatus
• Blue cod Parapercis colias
• The cod icefish family, Nototheniidae, including:
• Black cod Paranotothenia microlepidota
• Maori cod Paranotothenia magellanica
• Antarctic cod Dissostichus mawsoni

Rock cod, reef cod, and coral cod

Almost all the fish known as coral cod, reef cod or rock cod are also in order Perciformes. Most are better known as groupers, and belong to the family Serranidae. Others belong to the Nototheniidiae. Two exceptions are the Australasian red rock cod, which belongs to a different order (see below), and the fish known simply as the rock cod in New Zealand, Lotella rhacina, which as noted above actually is related to the true cod (it is a morid cod).

Order Scorpaeniformes

• Ling cod Ophiodon elongatus
• Red rock cod Scorpaena papillosa
  

Order Ophidiiformes:
The Tadpole cod family, Ranicipitidae, and the Eucla cod family, Euclichthyidae, were formerly classified in this order, but are now grouped with the Gadiformes.

Species marketed as cod
Some fish that do not have 'cod' in their names are sometimes sold as cod. Haddock and whiting belong in the same family, the Gadidae, as cod.

• Haddock Melanogrammus aeglefinus
• Whiting Merlangius merlangus

Cod trade
Cod has been an important economic commodity in an international market since the Viking period (around 800 AD). Norwegians used dried cod during their travels and soon a dried cod market developed in southern Europe. This market has lasted for more than 1000 years, passing through periods of Black Death, wars and other crisis and still is an important Norwegian fish trade. The Portuguese since the 15th century have been fishing cod in the North Atlantic and clipfish is widely eaten and appreciated in Portugal. The Basques also played an important role in the cod trade.

Apart from the long history this particular trade also differs from most other trade of fish by the location of the fishing grounds, far from large populations and without any domestic market. The large cod fisheries along the coast of North Norway (and in particular close to the Lofoten islands) have been developed almost uniquely for export, depending on sea transport of stockfish over large distances. Since the introduction of salt, dried salted cod (klippfisk) has also been exported. The trade operations and the sea transport were by the end of the 14th century taken over by the Hanseatic League, Bergen being the most important port of trade.

William Pitt the Elder, criticising the Treaty of Paris in Parliament, claimed that cod was British gold; and that it was folly to restore Newfoundland fishing rights to the French.

In the 17th and 18th centuries, the New World, especially in Massachusetts and Newfoundland, cod became a major commodity, forming triangular trade networks and cross-cultural exchanges. In the 20th century, Iceland re-emerged as a fishing power and entered the Cod Wars to gain control over the north Atlantic seas. In the late 20th and early 21st centuries, Cod fishing off the coast of Europe and America severely depleted Cod stocks there which has since become a major political issue as the necessity of restricting catches to allow fish populations to recover has run up against opposition from the fishing industry and politicians reluctant to approve any measures that will result in job losses. The 2006 Northwest Atlantic cod quota is set at 23,000 tons representing half the available stocks, while it is set to 473,000 tons for the Northeast Atlantic cod.