Tol Eressëa is a large island, where the mallorn trees come from. Its name may be translated from Elvish as the Lonely Island, for it lay originally in the middle of the Belegaer, far from any other landmasses.
Ulmo pushed it back and forth across Belegaer twice to transport the Elves to Aman. After that, it came to rest forever just off the eastern shore of that continent in the Bay of Eldamar, and was inhabited by the Teleri of Aman, until they left to Alqualondë.
In early versions of Tolkien's legendarium (see: The History of Middle-earth), the island was later visited by Ælfwine (or Eriol), an Englishman from the Middle Ages, which provided a framework for the tales that later became The Silmarillion.