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

 

onlooker

 noun
noun
BrE BrE//ˈɒnlʊkə(r)//
 
; NAmE NAmE//ˈɑːnlʊkər//
 
, NAmE//ˈɔːnlʊkər//
 
 
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a person who watches something that is happening but is not involved in it synonym bystander A crowd of onlookers gathered at the scene of the crash. Synonymswitnessobserver onlooker passer-by bystander eyewitnessThese are all words for a person who sees something happen.witness a person who sees something happen and is able to describe it to other people; a person who gives evidence in a court of law:Police have appealed for witnesses to the accident.observer a person who sees something happen:According to observers, the plane exploded shortly after take-off.onlooker a person who watches something that is happening but is not involved in it:A crowd of onlookers gathered at the scene of the crash.passer-by a person who is going past somebody/​something by chance, especially when something unexpected happens:Police asked passers-by if they had witnessed the accident.bystander a person who is near and can see what is happening when something such as an accident or fight takes place:Three innocent bystanders were killed in the crossfire.eyewitness a person who has seen a crime or accident and can describe it afterwards.Patterns a witness/​an observer/​an onlooker/​a passer-by/​a bystander/​an eyewitness sees something an observer/​an onlooker/​a passer-by/​a bystander witnesses something
See the Oxford Advanced American Dictionary entry: onlooker