/ˈbaɪstændər/
by·stand·er
BrE /ˈbaɪstændə(r)/
NAmE /ˈbaɪstændər/
a person who sees sth that is happening but is not involved
SYN onlooker
innocent bystanders at the scene of the accident
witnessobserver ◇ onlooker ◇ passer-by ◇ bystander ◇ eyewitness
These are all words for a person who sees sth happen.
witness a person who sees sth 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 sth happen：
According to observers, the plane exploded shortly after take-off.
onlooker a person who watches sth 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 sb/sth by chance, especially when sth 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 sth 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 sth
an observer/an onlooker/a passer-by/a bystander witnesses sth