also: かんじゃ
noun
A person who watches or observes something, such as an event, scene, or phenomenon. Often used in formal or literary contexts.
彼はただの観者としてその場にいた。
He was there merely as an observer.
観者の視点から見ると、状況は異なって見える。
From the observer's perspective, the situation looks different.
Compound of 観 (kan, 'to observe') and 者 (sha, 'person'). The reading かんじゃ is a variant, possibly influenced by rendaku or historical usage, but かんしゃ is standard.