Númenor is the great island kingdom destroyed in J.R.R. Tolkien's Akallabêth.
Númenor was an island realm created by the Valar for the Men who had supported the Elves during the wars against Morgoth. Its people became renowned for long life, seafaring, learning, and powerful kingdoms. The island lay in the western sea between Middle-earth and the Undying Lands.
Over time, Númenórean rulers became increasingly resentful of mortality. Sauron exploited that fear after being taken to Númenor as a prisoner, eventually corrupting King Ar-Pharazôn and encouraging an assault on the Undying Lands. The Valar appealed to Eru Ilúvatar, who changed the shape of the world and caused Númenor to be overwhelmed.
Akallabêth, meaning “The Downfall,” is the section of The Silmarillion that tells this catastrophe. Survivors led by Elendil founded the realms of Gondor and Arnor in Middle-earth. Númenor is therefore not the same as either kingdom, although it is their ancestral source.