onlooker

Etymology

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

noun

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. 1945 May and June, Cecil J. Allen, “British Locomotive Practice and Performance”, in Railway Magazine, page 152
   As a result, while the train was being shunted at Bombay, the buffers became locked, producing a situation most intriguing for the onlookers, but exasperating for the exalted passengers and the unhappy railway authorities. 1945 September and October, C. Hamilton Ellis, “Royal Trains—V”, in Railway Magazine, page 249