Númenor is the island kingdom founded by the Men who survived the destruction of Beleriand in Tolkien’s mythology.
The Valar raised Númenor from the sea as a reward for the Edain who had aided the Elves during the wars against Morgoth. The island lay in the Great Sea between Middle-earth and Aman, and its people became renowned for long life, seafaring, and learning.
Númenórean power eventually declined as its rulers grew jealous of the immortality associated with the Elves. Under King Ar-Pharazôn, the kingdom was drawn into conflict with Sauron and finally destroyed in the Second Age.
The survivors established realms in Middle-earth, including Gondor and Arnor. Númenor is therefore a major link between the First Age traditions and the political history described in The Lord of the Rings.