The island where Númenor was located in J.R.R. Tolkien’s legendarium was itself called Númenor.
Númenor was a great island kingdom in the Second Age, granted to the Men who had aided the Elves during the wars of the First Age. It lay in the Great Sea between Middle-earth and Aman, the realm associated with the Valar. The Númenóreans developed a powerful maritime culture and built an extensive kingdom.
The island’s capital was Armenelos, while its chief port was Rómenna. Númenor’s people were long-lived compared with other Men, but their relationship with the Eldar and the Valar deteriorated as fear of death grew. Under King Ar-Pharazôn, the Númenóreans challenged the authority of the Valar.
The island was destroyed in the Downfall of Númenor. Survivors led by Elendil reached Middle-earth and founded the realms of Arnor and Gondor. Númenor is therefore distinct from Aman, Tol Eressëa, and Balar, which are separate islands or lands in Tolkien’s geography.