Source : Webster's Revised Unabridged Dictionary (1913)

Banzai \Ban"zai"\, interj. [Jap. banzai, banzei, ten thousand
   years, forever.]
   Lit., May you live ten thousand years; -- used in salutation
   of the emperor and as a battle cry. [Japan]

Source : WordNet®

banzai
     n : a Japanese cheer of enthusiasm or triumph