Noun
One who estimates or judges based on visual inspection rather than detailed analysis.
A voyeur or inquisitive person.
An investigator who watches for signs of anything unusual or suspicious.
(pool) A secret accomplice who surreptitiously watches the competition and interferes by causing a distraction to prevent the opponent from getting a good shot.
Someone who stares at another in order to intimidate them.
Something eye-catching or worth noticing.
Source: en.wiktionary.org