The fictional sea separating Middle-earth from Aman in J.R.R. Tolkien’s legendarium is Belegaer. It is also known as the Great Sea or the Sundering Sea.
Belegaer lies west of Middle-earth and east of Aman, the realm that includes Valinor. During much of the First Age, the continent of Beleriand bordered its eastern shore. Important voyages across or along this sea include the journeys of the Elves, Eärendil, and later Númenóreans.
Belegaer should not be confused with the Bay of Belfalas, which is a smaller coastal feature on the southern shores of Middle-earth. Nor is it the Sea of Rhûn, an inland sea far to the east. Tolkien’s maps and narratives use several large bodies of water, but Belegaer is the principal oceanic boundary between the mortal lands and Aman.