Excalibur or Caliburn is the famous sword of legendary King Arthur, a powerful weapon, sometimes attributed with magical powers or associated with the rightful sovereignty of Great Britain.

Sometimes Excalibur and the Sword in the Stone are said to be the same weapon, but in most explanations they are considered to be separate. According to one of the most popular versions, Arthur received Excalibur from the Lady of the Lake, a mysterious figure who lived in an enchanted underwater realm. This event is depicted in this beautiful illustration from the Tales of Romance:

In Welsh, the sword is called Caledfwlch; in Cornish, the sword is called Calesvol; in Breton, the sword is called Kaledvoulc'h.

More Info: en.wikipedia.org