Bounce rate
Bounce rate measures the proportion of sessions where the visitor leaves the site after viewing only one page. In GA4 it is replaced by engagement rate, which accounts for time spent on the page before departure and provides a more nuanced picture of session quality.
In practice
A high bounce rate is not automatically negative: a definition page that immediately answers the question can see 80% bounce with an excellent satisfaction signal. The meaningful indicator is the combination of bounce rate and average session duration. A page with 75% bounce and 3 minutes of reading time outperforms one with 40% bounce and 20 seconds of session.