Avalon:
from the Welsh afallach, meaning "apples." Afallach is also a Welsh god of the Underworld and of healing, son of Nudd and brother to Gwyn ap Nudd. His island is called Ynys Afallach, "Isle of Apples." Related linguistically is the Irish Emhain Abhlach, "Plain of Apples," the home of Manannan mac Lir, trickster god of the sea and Otherworld. The association between apples and gods is not unique to the Celts--the Garden of the Hesperides are from Greek myth, the Apples of Youth from Norse, and the Garden of Eden from the Bible*. (Even today we say "an apple a day keeps the doctor away.")
The first direct association that Avalon has with King Arthur originates with Geoffrey of Monmouth's History of the Kings of Britain, ca. 1136, where he says that it is both the location of Excalibur's forging, and the island where Arthur was taken to be healed after the Battle of Camlann, and from which he will return when Britain needs him most. In the Vita Merlini (attributed to Geoffrey), this tradition is elaborated on by adding that it is to Morgan le Fey that Arthur is taken.
Later tradition also makes Avalon the home of the Holy Grail, due to the island's association with Glastonbury, which even in modern Welsh is called Ynys Afallach, though the older name is Ynys Witrin. The author of the Perlesvaus of the thirteenth century even claims that his version comes from "The Holy House of Avalon," which he identifies with Glastonbury. The tradition is that Joseph of Arimathea came to Glastonbury and started the Christianization of Britain in 70 AD. However, the origin of Avalon as a Celtic Otherworld cannot be denied.
I think it odd that as a child, I used to vacation in Avalon, NJ, and have since gone on to become a real King Arthur nut.
* For the record, the Bible never says that the fruit of the Tree of Knowledge is an apple. This is only a later tradition.