Númenor is Tolkien’s fictional island kingdom destroyed in the Second Age.
Númenor was an island realm granted by the Valar to the Edain after the War of Wrath. Its people, the Númenóreans, were given unusually long lives and became powerful sailors. Over time, however, their fear of death and desire for domination brought them into conflict with the divine order of the world.
Under King Ar-Pharazôn, Númenor invaded the Undying Lands. In response, Eru Ilúvatar changed the shape of the world and destroyed the island in the Downfall of Númenor. Elendil and his sons, including Isildur and Anárion, escaped and founded realms in Middle-earth.
Númenor is sometimes confused with Valinor, the blessed realm across the sea. Valinor was not the island kingdom destroyed in the Downfall. Númenórean history also provides important background for the ancestry of Aragorn.