High Sea

The open waters of the Great Sea, especially those that lay westward beyond the mouth of the Gulf of Lhûn into which the vessels of the Elves sailed from Mithlond, the Grey Havens.

During earlier ages, the Sea had reached the shores of Aman in the West, but after the Downfall of Númenor, Aman was taken away from the world, and the waves of the High Sea lapped other unknown shores in the distant west. Nonetheless, those with the blessing of the Valar could still sail the Straight Road across the Sea as it had been (as with the White Ship that carried the Ring-bearers out of Middle-earth) and so come to the Blessed Realm.

The term 'high sea' or 'high seas' in modern usage implies a part of the ocean that falls outside the control of any country. That clearly cannot be the literal meaning intended by Tolkien, but he perhaps chose the term to suggest the Great Sea as being beyond the influence of the mortal lands of Middle-earth (thus, for the Elves, permitting a journey into the West). As we only have a single instance of the term, however, it is difficult to be sure precisely what its intended meaning might have been.