The fictional island kingdom central to J.R.R. Tolkien’s Second Age history is Númenor.
Númenor was an island kingdom established in the Second Age for the Men who had aided the Elves during the wars of the First Age. Its people were known as the Númenóreans, or Dúnedain, and they were granted unusually long lives compared with other Men.
The kingdom’s history combines maritime exploration, political expansion, and a growing resentment of mortality. Under King Ar-Pharazôn, Númenor challenged the authority of the Valar and attempted to reach Aman. Tolkien’s account culminates in the island’s destruction and submergence, after which survivors founded realms in Middle-earth.
Númenor is sometimes confused with Valinor, the land of the Valar, or Beleriand, a region destroyed at the end of the First Age. Númenor is an island kingdom populated by Men; it is not one of the Elven realms of Tolkien’s earlier histories.