1. (n.) One set to watch; a person who keeps guard; a guard; a sentinel.

2. (n.) Specifically, one who guards a building, or the streets of a city, by night.

Argus Charley airplane spotter fire patrolman fire warden fireguard forward observer lookout lookout man night watchman patrol patroller patrolman picket roundsman scout sentinel sentry spotter vedette ward watch watcher watchkeeper