Waiters, keen observers of humanity that they are, are catching on to this; in one poll, a full 30% said they didn't believe the job they did had any impact on the tips they received.
2016年12月四级真题（第一套）阅读 Section C
英语六级真题
The adult observers were able to reliably discern an assortment of emotions on the babies faces.
2019年12月六级真题（第一套）听力 Section C
柯林斯高阶英汉双解学习词典释义
N-COUNT
观察者；观测者；目击者
You can refer to someone who sees or notices something as an observer .
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 so that they can tell other people about it.