The island kingdom of the Dúnedain was called Númenor.
Númenor was created in the Second Age as a reward for the Men who had helped the Elves and the Valar during the wars of the First Age. The island lay in the western sea between Middle-earth and Aman, and its people became renowned for long life, seafaring, and learning.
Over time, Númenórean pride and fear of death grew stronger. Under King Ar-Pharazôn, the kingdom challenged the power of the Valar and attempted to reach the Undying Lands. In Tolkien’s mythology, this rebellion led to the island’s destruction and submersion in the cataclysm known as the Downfall of Númenor.
Gondor and Arnor were later kingdoms founded by Númenórean survivors in Middle-earth. They are related to Númenor, but neither name identifies the original island kingdom.