The Gulf of Guinea is the part of the Atlantic Ocean southwest of Africa. The intersection of the Equator and Prime Meridian (zero degrees latitude and longitude ) is in the gulf. According to the International Hydrographic Organization, the Gulf's oceanic border is the rhumb line that runs from Cape Palmas in Liberia to Cape Lopez in Gabon (IHO Special Publication 23, Limits of Oceans and Seas, 3rd ed. (1953), #34).
The Gulf derives its name from the former names of the coasts of Africa. The south coast of West Africa, north of the Gulf of Guinea, was historically called "Upper Guinea." The west coast of Southern Africa, to the east, was historically called "Lower Guinea." The name "Guinea" is still attached to the names of three countries in Africa: Guinea, Guinea-Bissau, and Equatorial Guinea, as well as New Guinea in Melanesia.
The Niger River in particular deposited organic sediments out to sea over millions of years which became crude oil. The Gulf of Guinea region, along with the Congo River delta and Angola further south, are expected to provide around a quarter of the United States' oil imports by 2015. This region is now regarded as one of the world's top oil and gas exploration hotspots.
The name Guinea is usually said to have been a corrupt form of the name Ghana, picked up by the Portuguese in the Maghrib. The present writer finds this unacceptable. The name Guinea has been in use both in the Maghrib and in Europe long before Prince Henry’s time. For example, on a map dated about 1320 by the Genoese cartographer Giovanni di Carignano, who got his information about Africa from a fellowcountryman in Sijilmasa [ancient trading city in North Africa], we find Gunuia, and in the Catalan atlas of 1375 as Ginyia. A passage in Leo [Africanus] (vol. III, 822) points to Guinea having been a corrupt form of Djenne [2,000 year old city in central Mali on Niger river], less famous than Ghana but nevertheless for many centuries famed in the Maghrib as a great market and a seat of learning. The relevant passage reads: “The Kingdom of Ghinea . . . called by the merchants of our nation Gheneoa, by the natural inhabitants thereof Genni and by the Portugals and other people of Europe Ghinea.” But it seems more probable that Guinea derives from aguinaou, the Berber for Negro. Marrakech [city in southeastern Morocco] has a gate, built in the twelfth century, called the Bab Aguinaou, the Gate of the Negro (Delafosse, Haut-Sénégal-Niger, II, 277-278). The modern application of the name Guinea to the coast dates only from 1481. In that year the Portuguese built a fort, São de Mina (modern day Elmina), on the Gold Coast, and their king, John II, was permitted by the Pope [Sixtus II or Innocent VIII] to style himself Lord of Guinea, a title that survived until the recent extinction of the monarchy.