Watcher (noun)

1. A person who watches someone or something, especially as a guard.
2. A person who keeps a watchful eye on someone or something.
3. A person who is vigilant and attentive.

Origin:

Late middle english (denoting a person who keeps watch, especially as part of a guard): from watch + -er.

Examples:

1. She is a trained watcher and can detect any suspicious activity.
2. The police appointed watchers to monitor the building.
3. The watcher was keeping an eye on the children as they played in the park.
4. The bird-watcher was waiting for the rare species to show up.
5. The political watcher was closely following the developments in the election.