From WordNet (r) 3.0 (2006) [wn]:
Halloween
n 1: the evening before All Saints' Day; often devoted to pranks
played by young people [syn: Halloween, Hallowe'en,
Allhallows Eve]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]