Tolkien calls the destruction of Númenor “Akallabêth.”
Akallabêth is the Quenya name for the account of Númenor’s downfall, and it is also the title of the section in The Silmarillion that tells the story. Númenor was an island kingdom granted to the Edain after the War of Wrath. Over time, its people became increasingly troubled by mortality and jealous of the immortal Elves.
Under King Ar-Pharazôn, Númenor challenged the power of the Valar and sailed against Aman. Sauron had been taken to the island as a prisoner, but he corrupted the king and encouraged rebellion against the ban on sailing west. The resulting catastrophe caused Númenor to be submerged, while Elendil and his sons escaped to Middle-earth.
The event is sometimes called the Downfall of Númenor in English. “Akallabêth” is more specific: it is the title and Quenya name associated with this legend. The story has clear thematic parallels with Atlantis, but Tolkien’s account belongs to his own mythology.