1. A spectator.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• unlooked,
• onlook,
• unlookers,
• unlocker,
• inlooker,
• knlooker,
• lnlooker,
• pnlooker,
• 0nlooker,
• 9nlooker,
• oblooker,
• omlooker,
• ojlooker,
• ohlooker,
• onkooker,
• onpooker,
• onloooker,
• onlookerd,
• onloookers,
• onlookers

The adjective form of "onlooker" is "onlooking".

The plural form of "onlooker" is "onlookers"

The singular form of the word "onlooker" is "onlooker". The word does not change when referring to a single person.