The percentage of all of your website visitors who land on a page on your website and then leave your site after viewing that single page. Importantly, bounce rate has slightly different definitions depending on the analytics tool you’re using. In Google Analytics, for example, bounce rate is “single-page sessions divided by all sessions, or the percentage of all sessions on your site in which users viewed only a single page” (Source: Google).
It could also be the average across all pages of the percent of people who viewed a specific page and then left.
A high bounce rate doesn’t always mean that users don’t like your site. For example, if you write a really thorough blog post that’s intended to answer a ton of FAQs, people could spend a significant amount of time reading the page and then leave because the website has answered all of the questions they had.