noun
Refers to someone who watches an event without participating, often with a nuance of passive observation.
事故のとき、多くの傍観者が集まった。
Many onlookers gathered at the time of the accident.
彼は傍観者としてその議論を見ていた。
He watched the argument as a bystander.
Compound of 傍 (side, nearby) + 観 (view, observe) + 者 (person). The word literally means 'person who observes from the side'.