The etymology of "Guinea" is uncertain. The English term Guinea comes directly from the Portuguese word Guiné, which emerged in the mid-15th century to refer to the lands inhabited by the Guineus, a generic term used by the Portuguese to refer to the 'black' African peoples living south of the Senegal River (in contrast to the 'tawny' Sanhaja Berbers, north of it, whom they called Azenegues). The term "Guinea" is extensively used in the 1453 chronicle of Gomes Eanes de Zurara. King John II of Portugal took up the title of Senhor da Guiné (Lord of Guinea) from 1483.
It is believed the Portuguese borrowed Guineus from the Berber term Ghinawen (sometimes Arabized as Guinauha or Genewah) meaning "the burnt people" (analogous to the Classical Greek Aithiops, "of the burned face"). The Berber terms "aginaw" and "Akal n-Iguinawen" mean "black" and "land of the blacks", respectively.
A competing theory, first forwarded by Leo Africanus in 1526, claims that 'Guinea' is derived from Djenné (which he refers to as Gheneo, Genni and Ghinea), the great interior commercial city on the Upper Niger River. Djenné dominated the gold and salt trade across West Africa, from the 11th century (fall of Ghana) until the 13th century (when the Mali invasion disrupted its routes and redirected trade to Timbuktu, hitherto just a small Djenné outpost). It is during the period of Djenné dominance that the term Genewah really comes forward into usage in Arab sources (al-Sudan - Arabic for "blacks" - is used more commonly before).
Other theories try to connect "Guinea" to "Ghana", but this is less certain. The Ghana Empire is named after the Medieval trading city of Ghanah mentioned already by 11th-century Arab geographers (e.g. al-Bakri), but it is used distinctly from Genewah by Arab sources (e.g. they would say "Ghanah in the country of Genewah"). Conversely, it remains possible that both Ghana and Djenné themselves owe their original city names to the Berber appellation for the blacks that lived there. A possible reconciliation of the theories is that the Berber Ghinawen (blacks) was the source of the Djenné (city), which in turn gave rise to the Arabic Genewah (land dominated by that city), which finally made it into the Portuguese Guiné.