watcher in WordNet English dictionary

1. a close observer; someone who looks at something (such as an exhibition of some kind)
   "the spectators applauded the performance"; "television viewers"; "sky watchers discovered a new star"
   spectator  witness  viewer  watcher  looker 
   
2. a guard who keeps watch
   watchman  watcher  security guard 
   
3. a person who keeps a devotional vigil by a sick bed or by a dead body
   watcher