From Tolkien Gateway
The Bent World was a name describing the condition of Arda after the destruction of Númenor and the removal of Valinor in the late Second Age; after this, the world was made 'bent' (or 'round') and only the Elves could still follow the Straight Road to the Undying Lands.
Other versions of the legendarium
- See also: Round World version of the Silmarillion.
In Tolkien's writings as published in the later volumes of The History of Middle-earth, Arda was a round world from its beginning, and the idea of a flat world was instead an invention of the Númenóreans. Thus, in this alternate account, the world would not need to be bent at the time of Númenor's downfall and Aman's removal from the physical world.[source?]