Source : Webster's Revised Unabridged Dictionary (1913)

Excalibur \Ex*cal"i*bur\, n.
   The name of King Arthur's mythical sword. [Written also
   {Excalibar}, {Excalibor}, {Escalibar}, and {Caliburn}.]
   --Tennyson.

Source : WordNet®

Excalibur
     n : the legendary sword of King Arthur