onlooker

noun

Etymology: From on + look + -er, probably from the verb look on, but compare with onlook.

1. A spectator; someone looks on or watches, without becoming involved or participating.

   “I wasn’t involved in the fight; I was only an onlooker.”

   “When the right-away was given, Driver Gibson would give a sonorous blast on Cardean's deep-toned hooter, and amid a flurry of swirling steam the train would move majestically out, with nearly half the city of Carlisle—or so it would appear—as onlookers on the platform.”