Meaning of “passer-by” - Learner’s Dictionary

passer-by

noun [ C ] uk us /ˌpɑːsəˈbaɪ/ (plural passers-by)

someone who is walking past something by chance:

Police were alerted by a passer-by who saw the accident.

(Definition of “passer-by” from the Cambridge Learner’s Dictionary © Cambridge University Press)