Extended definition
Time until the first visible element appears on the page (any text/image). Target: under 1.8 seconds.
Context and application
FCP vs LCP: FCP is when you see the FIRST element (could be a small logo in header), LCP is when you see the LARGEST relevant element (usually hero image or first paragraph). Fast FCP + slow LCP = user sees “something” quickly, but waits for main content. Common optimizations are common for both: critical CSS inline, eliminate render-blocking JS, preload for critical resources. FCP is a useful indicator in debug — if FCP is good and LCP is bad, the problem is specific to the LCP element (usually image or font).