Cumulative Layout Shift
Also known as: CLS
Cumulative Layout Shift measures how much a page's content moves unexpectedly while it loads.
In full
Cumulative Layout Shift, or CLS, quantifies unexpected movement of visible content during a page's life, the experience of reaching for a button and having the page shift it. Google considers a score under 0.1 good and over 0.25 poor. Every common cause is preventable: images and embeds without declared width and height give the browser no way to reserve space, adverts and iframes injected into the flow push content down, web fonts swapping in at a different size shift the text below them, and banners inserted above existing content move everything a visitor was already reading.
Why it matters
It is the Core Web Vital users notice most viscerally, because it causes mis-taps rather than merely delay, and it is usually the cheapest of the three to fix.