A passer-by is a person who is walking past someone or something.
例句
A passer-by described what he saw moments after the car bomb had exploded...一位过路人描述了汽车炸弹爆炸后他看到的情形。I went and sat in a cafe and watched the passers-by.我走进一家咖啡馆坐下，望着来往的路人。
英汉词典释义
n.
过路人,经过者
A passer-by called the emergency services (= the ambulance/fire/police services).一名路人拨打了紧急救险服务电话。