The fictional island kingdom destroyed by a great wave in J.R.R. Tolkien’s Second Age history is Númenor.
Númenor was given to the Men who had aided the Elves during the War of Wrath at the end of the First Age. Its people became powerful mariners, and the island kingdom stood between Middle-earth and Aman. Over time, many Númenóreans became resentful of their mortality and increasingly challenged the ban against sailing to the Undying Lands.
King Ar-Pharazôn eventually sailed west with a great fleet, influenced by Sauron, who had been taken to Númenor as a prisoner. The Valar appealed to Eru Ilúvatar, who changed the shape of the world and destroyed Númenor. Elendil and his sons survived and founded the kingdoms of Arnor and Gondor in Middle-earth. Númenor is therefore also central to the ancestry of Aragorn.