1. Mythical sword. Excalibur is the sword of King Arthur. In some accounts it is the same as the sword that Arthur alone was able to draw from the stone, thus denoting his kingship. In other cases, it is a different sword, given him by the Lady of the Lake. On his death, it was eventually returned to the Lady of the lake by Sir Bedivere.
3. Comic. Excalibur is an older Marvel comic about a group of cross-dimensional travelers. The leader of which is Capt. Britain from which the comics name is derived.