Númenor is the island kingdom destroyed before the Third Age in Tolkien’s mythology.
Númenor was an island realm in the Second Age, given to Men who had aided the Elves during the wars against Morgoth. Its people lived longer than other Men, and its kings became increasingly powerful and proud. The kingdom’s capital was Armenelos.
Under King Ar-Pharazôn, Númenor challenged the Valar and sought to escape death. Its destruction came when the king’s fleet attempted to invade Aman. Eru Ilúvatar changed the shape of the world, causing the island to sink beneath the sea. The survivors founded the realms of Arnor and Gondor in Middle-earth.
Númenor is sometimes called Westernesse in English translations and related texts. It should not be confused with Gondor, which was one of the realms founded by Númenórean exiles after the island’s fall.