Source : Webster's Revised Unabridged Dictionary (1913)

Halloween \Hal`low*een"\, n.
   The evening preceding Allhallows or All Saints' Day. [Scot.]

Source : WordNet®

Halloween
     n : the evening before All Saints' Day; often devoted to pranks
         played by young people [syn: {Hallowe'en}, {Allhallows
         Eve}]