Definition
The canonical answer.
Core Web Vitals are Google’s field-measured user experience metrics: Largest Contentful Paint (loading), Interaction to Next Paint (responsiveness), and Cumulative Layout Shift (visual stability). They are measured from real visitors, so lab scores can pass while the field fails, and the field data is what counts.
Worked example
In practice.
A page loads fast in a lab test but real users on mid-range phones see images shove the layout around as they load. The field CLS fails. The fix is intrinsic width and height on media, reserved space for embeds, and fonts that swap without reflowing the page.
Where this work happens: Website Optimization
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