Bounce rate is a metric used in web traffic analysis that represents the percentage of visitors who enter a website and then leave ("bounce") rather than continuing to view other pages within the same site. It is calculated by dividing the number of single-page visits by the total number of visits.

For example, if a website receives 100 visitors in a day, and 40 of those visitors leave after viewing only one page, the bounce rate for that day would be 40%.

A high bounce rate can indicate that a website's landing page is not relevant or attractive to visitors, or that the site is difficult to navigate. Conversely, a low bounce rate suggests that visitors are finding the website useful and engaging, prompting them to explore more pages on the site.

It's important to note that a high bounce rate isn't always a bad thing. For instance, if a user lands on a blog post, reads the entire article, and then leaves, that would still be considered a bounce even though the user found what they were looking for and were satisfied with their visit.

In the context of A/B testing, bounce rate can be a key metric to understand how changes to a webpage or user interface impact user behavior. For example, if version A of a webpage has a higher bounce rate than version B, it could indicate that version B provides a better user experience.