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
Excalibur is by no means the only weapon associated with Arthur, nor the only sword. Welsh tradition also knew of a dagger named Carnwennan and a spear named Rhongomyniad that belonged to him. Carnwennan ("little white-hilt") first appears in "Culhwch and Olwen", where Arthur uses it to slice the witch Orddu in half. Rhongomyniad ("spear" + "striker, slayer") is also mentioned in "Culhwch", although only in passing; it appears as simply "Ron" ("spear") in Geoffrey's "Historia".