The one of the most common questions regarding using Google Analytics is the definition of bounce rate. In some tutorials you could find the information that bounce rate is the percentage of visits that immediately left your site, however this definition is not right since there is no time boundary that would define it.
Bounce rate is the percentage of single-page, no-event, no-goal, no-transaction visits, regardless the time spent.
The value of bounce rate is most often mentioned with dimensions such as “source / medium” or “landing page”. Bounce rate with dimension “source / medium” gives you the percentage of bounced visits coming from a particular source and with dimension “landing page” the percentage of bounced visits landing on a particular page.
In general, the lower the bounce rate the better, however it is not always the case. The metric is, besides the quality of traffic source, influenced by many on-page factors, therefore is always has to be eveluated in the context. In extreme, if you would squeeze all the important content in one landing page (i.e. there is no reason for the user to further continue to your site) the bounce race could be as high as 100%, but you could still end up with satisfied users.
To introduce a time factor into bounce rate, there is a solution in Google Analytics that requires firing an event after 10-15 seconds. This will ‘unbounce’ every visit after set time.