The term Bounce Rate has two main definitions depending on the context. In email marketing, it refers to the percentage of emails sent that are not delivered to the recipient, either because the address does not exist or there is some other technical problem.
In the context of web analytics, Bounce Rate refers to the percentage of visitors who enter a website and then leave («bounce») instead of proceeding to view other pages within the same site. A high Bounce Rate may indicate content, design or usability problems.