Definition of Halloween noun from the Oxford Advanced Learner's Dictionary

 

Halloween

 noun
(also Hallowe’en)noun
BrE BrE//ˌhæləʊˈiːn//
 
; NAmE NAmE//ˌhæloʊˈiːn//
 
[countable, uncountable]
 
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the night of 31st October when it was believed in the past that dead people appeared from their graves, and which is now celebrated in the US, Canada and Britain by children who dress as ghosts, witches, etc. see also trick or treat Word Origin late 18th cent.: contraction of All Hallow Even.
See the Oxford Advanced American Dictionary entry: Halloween