1. (n.) A soldier placed on guard; a sentinel.

2. (n.) Guard; watch, as by a sentinel.

Argus Cassandra Charley Jeremiah admonisher airplane spotter cautioner fire patrolman fire warden fireguard flagman forward observer lighthouse keeper lookout lookout man monitor night watchman patrol patroller patrolman picket prophet of doom roundsman scout sentinel signaler signalman spotter vedette ward warner watch watcher watchkeeper watchman