To watch or monitor something or someone carefully, often to ensure safety or proper behavior.
This phrase dates back to the 16th century and uses the metaphor of the eye as a vigilant watcher. The singular 'eye' rather than 'eyes' emphasizes focused, concentrated attention on a specific target.
Interestingly, this phrase spawned the related expression 'keep an eye out for,' which shifts from monitoring something you can already see to watching for something that might appear. The eye has been a symbol of vigilance across cultures for millennia, appearing in everything from the Eye of Horus to the all-seeing eye on the dollar bill.
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