The island kingdom west of Middle-earth in J.R.R. Tolkien’s legendarium is Númenor.
Númenor was established as a gift to the Edain after the end of the First Age. Its people, the Númenóreans, were granted a long life and great knowledge, though they were forbidden to sail west to the Undying Lands. The island lay in the Great Sea between Middle-earth and Aman.
Over time, fear of death and resentment of the ban contributed to Númenor’s decline. Under King Ar-Pharazôn, the Númenóreans challenged the powers of the West. Their island was destroyed and submerged in the event later called the Downfall of Númenor.
Númenor is not the same as Valinor, which is a region of Aman, or Tol Eressëa, an island near Aman. Tolkien modeled parts of the Númenórean story on flood and Atlantis traditions while giving it a distinctive place in his mythology.