An onlooker is someone who watches an event take place but does not take part in it.
例句
A handful of onlookers stand in the field watching.少数几个旁观者站在现场观看。
英汉词典释义
n.
旁观者, 观看者
She blew a kiss to the onlookers.她向旁观的人群送去一个飞吻。A crowd of onlookers formed around the fight.旁观者聚集起来围观斗殴。After the crash,onlookers said that the plane had suddenly dropped down out of the sky.飞机坠毁后旁观者说飞机是突然从天上掉下来的。