Belegaer was also called the Great Sea, the Western Sea and the Great Water. It lay between western Middle-earth and Aman, the Undying Lands.

In SA32, the Island of Númenor was raised out of this sea and was given to the men that had stayed faithful in the wars of the Valar with Morgoth. After Númenor was swallowed beneath the Sea in SA3319 the world was altered so that only Elves could sail across Belegaer to Aman.
Encyclopedia entry originally written by PotbellyHairyfoot