observer
noun
[ əbˈzəːvə ]
• a person who watches or notices something.
• "to a casual observer, he was at peace"
Similar:
spectator,
onlooker,
watcher,
looker-on,
fly on the wall,
viewer,
witness,
eyewitness,
bystander,
sightseer,
commentator,
reporter,
blogger,
monitor,
rubberneck,
beholder,
