watcher
nounUK /ˈwɒtʃə/
US /ˈwɑːtʃər/
A watcher is a person who looks at something for a long time.
The bird watcher stood quietly by the lake.
OTHER MEANINGS
2. A person who keeps guard over a place or people.
The night watcher checked the gates every hour.
3. A device or program that monitors something.
The website watcher alerts us when prices drop.
4. Someone who regularly follows an event or group.
She is a close watcher of local politics.