You can refer to someone who sees or notices something as an observer.
近义词
witness
双语例句
例:
A casual observer would have taken them to be three men out for an evening stroll.
乍一看会以为这三个男人是晚上出来闲逛的。
例:
He argues that truly objective science is impossible: the observer affects the system he or she observes.
他争辩说完全客观的科学是不存在的:观察者会对其观察的系统产生影响。
2.?[C 可数名词] 观察家
An observer is someone who studies current events and situations, especially in order to comment on them and predict what will happen next.
双语例句
例:
Observers say the events of the weekend seem to have increased support for the opposition.
观察家们说周末事件似乎加大了对反对派的支持。
3.?[C 可数名词] 观察员
An observer is a person who is sent to observe an important event or situation, especially in order to make sure it happens as it should, or to tell other people about it.
双语例句
例:
The president suggested that a UN observer should attend the conference.