Excalibur
Geoffrey of Monmouth, in his "Historia Regum Britanniae" ("The History of the Kings of Britain", c. 1136), Latinised the name of Arthur's sword as "Caliburnus" (potentially influenced by the Medieval Latin spelling "calibs" of Classical Latin "chalybs", from Greek "chályps" ["χάλυψ"] "steel"). Most Celticists consider Geoffrey's "Caliburnus" to be derivative of a lost Old Welsh text in which "bwlch" (Old Welsh "bulc[h]") had not yet been lenited to "fwlch" (Middle Welsh "vwlch" or "uwlch"). In the late 15th/early 16th-century Middle Cornish play "Beunans Ke", Arthur's sword is called "Calesvol", which is etymologically an exact Middle Cornish cognate of the Welsh "Caledfwlch". It is unclear if the name was borrowed from the Welsh (if so, it must have been an early loan, for phonological reasons), or represents an early, pan-Brittonic traditional name for Arthur's sword.
Excalibur
However, in what is called the Post-Vulgate Cycle, Excalibur was given to Arthur by the Lady of the Lake sometime after he began to reign. In the Vulgate "Mort Artu", Arthur is at the brink of death and so orders Griflet to throw the sword into the enchanted lake; after two failed attempts (as he felt such a great sword should not be thrown away), Griflet finally complies with the wounded king's request and a hand emerges from the lake to catch it. This tale becomes attached to Bedivere instead of Griflet in Malory and the English tradition. Malory records both versions of the legend in his "Le Morte d'Arthur", naming both swords as Excalibur.