noun
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.
傍観者 implies a passive onlooker, often with a nuance of not intervening, while 観察者 is a more neutral or active observer.
Compound of 観察 (kansatsu, 'observation') + 者 (sha, 'person').