the.com/observer

means A person who watches or notices things carefully, often without taking part themselves.

from From Latin observare — ob- ('toward') plus servare ('to keep, watch, guard'). The same servare root sits inside 'preserve' and 'conserve,' so an observer is etymologically a kind of keeper: someone who keeps their eyes fixed on something, guarding it with attention rather than hands. It passed through Old French observer into English by the 14th century, originally carrying a sense closer to 'comply with' or 'keep a rule' (as in observing a law or custom) before settling into the watcher-who-notices sense we use today.