This is the name of Arthur's sword in Geoffrey of Monmouth
's The History of the Kings of Britain
, in which he says it is forged on the Isle of Avalon
. It is one of Arthur's three weapons according to Geoffrey, the others being his spear Ron
and his shield Pridwen or Prydwen
The name is derived from the Welsh name for the sword, Caledfwlch, which in turn is derived from the Irish Caladbholg, the sword of Cuchulainn. "Caladbholg" means "Hard Lightning."
Sir Thomas Malory called it Excalibur, which he says means "Cut Steel."