This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. One that looks on; a spectator.

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

"If it were at all a likeness, the woman who gazed frankly out upon the onlooker from the card-mount must have been a striking creature indeed."

looker    watcher    spectator    viewer    witness   

looker-on    spectator   

bystander    spectator    listener    observer    outsider