Definition of trick or treat from the Oxford Advanced Learner's Dictionary

 

trick or treat

 
BrE
 
; NAmE
 
 
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a traditional activity at Halloween, in which children dress in costumes and visit houses. At each house they say 'Trick or treat?' This means that they threaten to play a 'trick', or joke, on the people in the house unless they are given a 'treat', e.g. sweets or money. The practice of 'trick or treat' began in the US in the 1930s but is now common in Britain also.
See the Oxford Advanced American Dictionary entry: trick or treat