A person who watches or examines something carefully, often in a scientific, formal, or neutral context.
彼は会議で観察者として参加した。
He participated in the meeting as an observer.
観察者は鳥の行動を記録した。
The observer recorded the bird's behavior.
Compound of 観察 (kansatsu, 'observation') + 者 (sha, 'person').