Excalibur

King Arthur's magic sword.

The first mention of Excalibur is in "Culhwch ac Olwen", composed in tenth or eleventh century. There it's called Caladfwlch, a name which seems derived from Fergus mac Róich's sword Caladbolg--"hard lightning". "Culhwch ac Olwen" has a number of names borrowed from the Ulster Cycle--

Crychwr the son of Nes, [Conchobor mac Nessa] and Cubert the son of Daere [Curoi mac Dairi], and Percos the son of Poch [Fergus mac Róich]

and so it isn't surprising that the name of Arthur's sword could have an Irish origin.

In Geoffrey of Monmouth, this becomes Caliburnus, which Geoffrey says was made on the isle of Avalon.

In the French tales, this sword becomes Excalibur (or Escalibor). In the Vulgate, this is given a false etymology, with "Excalibur" explained as "a Hebrew word that means in French 'cuts through iron and steel and wood'" and is identified with the Sword in the Stone.

In Malory, there are two swords; first is the Sword in the Stone; this breaks during a battle with King Pellinore. Merlin then takes Arthur to the Lady of the Lake, who gives him Excalibur. The association between Excalibur and water may hearken back to Geoffrey's assertion that Caliburnus was made on the isle of Avalon; Malory knew of several traditions beyond that of the Vulgate and post-Vulgate, and may have been attempting a syncretic text with regards to the sword.

In Malory, Excalibur's magic extends to its scabbard; according to Merlin, the wearer of the scabbard cannot be wounded. The sword is stolen by Morgan le Fay and replaced by a copy.

Finally, after the battle of Camlann, Arthur instructs Bedevere to throw the sword into a neary-by body of water. Twice Bedevere pretends to throw it in, each time lying to Arthur. Each time Arthur sends him back. On the third time, Bedevere throws the sword; a hand--presumably the Lady of the Lake--comes out of the lake, swings the sword three times, and returns into the water with the sword.

Instead of the Irish derivation, Lacy connects the name Excalibur to the Greek chalybs, meaning "steel", derived from a tribe in Anatolia. Either may be correct, of course.

SOURCES
Gantz, Jeffrey. "Culhwch and Olwen". notes. The Mabinogion. NY: Penguin, 1972.

Lacy, Norris J. "Excalibur" The New Arthurian Encyclopedia.

----- The Lancelot-Grail Cycle vol. I. p.219.