Shows the percentage of visits that contain a single hit.
Bounce rate uses the Bounces metric and is calculated as:
Bounces divided by Entries
Bounce Rate does not include visits where multiple actions occurred on a single page. For example, a visit with a video view on a single page is a Single Access but not a Bounce.
Existing implementations can sometimes contain a calculated metric that differs from the Analytics default metric. Check the calculated metric definition to make sure there are no differences.
See this Knowledgebase article for more information.