watcher

(noun) a close observer; someone who looks at something (such as an exhibition of some kind). Synonyms: spectator, witness, viewer, looker. Examples: "The spectators applauded the performance." "Television viewers." "Sky watchers discovered a new star."

(noun) a guard who keeps watch. Synonyms: watchman, security guard.

(noun) a person who keeps a devotional vigil by a sick bed or by a dead body.