The Collaborative International Dictionary of English v.0.48:
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] [Webster 1913 Suppl.]WordNet (r) 3.0 (2006):
banzai n 1: a Japanese cheer of enthusiasm or triumph