Númenor is a fictional location from J. R. R. Tolkien's universe of Middle-earth and is intended to be his version of Atlantis. From the Quenya Númenórë: "West-land", which Tolkien translated as "Westernesse".
Númenor is the kingdom of the Dúnedain, located on an island in the Great Sea, between Middle-earth and Aman. The land was brought up from the sea as a gift to Men. It was also called Elenna, the land of the Star: Men were led to it by the star Eärendil and the island itself was in the shape of a five-pointed star. At the center of the island was a mountain named the Meneltarma, which the Dúnedain used as a temple to Ilúvatar.
Elros son of Eärendil was the first King of Númenor, taking the name of Tar-Minyatur ("First King"). Under his rule (year 32 to 442 of the Second Age), and those of his descendants, the Men rose to become a major race. Their first ships sailed from Númenor to Middle-earth in the year 600 of the Second Age.
The Númenóreans were forbidden by the Valar from sailing so far westward that Númenor was no longer visible, for fear that they would come upon the Undying Lands, to which Men could not come. Over time the Númenóreans came to resent the Ban of the Valar and to rebel against their authority, seeking the everlasting life that they believed was begrudged them. Few (the "Faithful") remained loyal to the Valar and friendly to the Elves.
In the year 3255 of the Second Age, the 25th king, Ar-Pharazôn, sailed to Middle-earth. Seeing the might of Númenor, Sauron agreed to be the king's captive, and he was brought back to Númenor. Sauron soon became an advisor to the King, and suggested that he defy the Ban. In the year 3319 of the Second Age, Ar-Pharazôn set foot on Aman; as a result, the world was changed and Númenor sank beneath the waves.
Elendil, son of the leader of the Faithful during the reign of Ar-Pharazôn, his sons and his followers had foreseen the disaster that was to befall Númenor, and they had set sail before the island fell. They landed in Middle-earth, and founded the kingdoms of Arnor and Gondor.
After its fall Númenor was called Atalantë, meaning The Downfallen, in the Quenya language. (The similarity with Atlantis is obvious, although Tolkien described his invention of the name as a happy accident when he realised that the Quenya root meaning "fallen" could be incorporated into a name referring to Númenor.)
The story of the rise and downfall of Númenor is told in the Akallabêth.