The Collaborative International Dictionary of English v.0.48:

Halloween \Hal`low*een"\ (h[a^]l`l[-o]*[=e]n"), n.
   The evening preceding Allhallows or All Saints' Day (November
   1); also the entire day, October 31. It is often marked by
   parties or celebrations, and sometimes by pranks played by
   young people. [Scot.] --Burns.

   Syn: Hallowe'en, Allhallows Eve.
        [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

Halloween
    n 1: the evening before All Saints' Day; often devoted to pranks
         played by young people [syn: Halloween, Hallowe'en,
         Allhallows Eve]