Bounce rate came back and it still is not what you think
Bounce rate returned to GA4 after being conspicuously absent, and it is defined differently from the number everyone remembers.
Universal Analytics: a bounce was a session with a single interaction hit. One page, no events, no matter how long they stayed. A visitor who read a 3,000-word article for eleven minutes and left was a bounce.
GA4: bounce rate is simply 100% minus engagement rate. A session is engaged if it lasts longer than 10 seconds, or fires a key event, or has two or more page views.
Same name, different definition, and not comparable across the two systems. Any chart that spans the migration is comparing two different things.
Why the new one is better, mostly
The old definition punished pages that did their job. A single-page answer that satisfied the visitor completely scored identically to a page nobody could use.
The new definition at least notices that someone stayed. It is a better proxy for "did this page work".
Why it is still weak
Ten seconds is arbitrary. It is a threshold — adjustable between 10 and 60 seconds in the property settings — pretending to be a fact about human attention. A visitor who left after 11 seconds is engaged; one who read carefully for 9 is not. Change the setting and your bounce rate changes with no change in behaviour.
It is a rate, so it hides volume. A page with 4 sessions and 100% engagement outranks one with 4,000 sessions and 70%, on a chart sorted by rate.
It says nothing about outcome. Engagement is not conversion, and optimising for it can move you away from conversion — a longer, more confusing checkout is more engaging by this definition.
What to use instead
For content: scroll depth plus time, or a read event fired at a meaningful threshold. Both are more honest about whether the page was consumed.
For commercial pages: conversion rate, and the funnel step where people leave. Engagement rate on a landing page is a distraction from the question of whether the form was started.
For a genuine "is this page failing" signal: entrances, conversion rate, and exit rate together. All three, per page. Bounce rate on its own has never answered a question worth asking.