1) Web Analytics Bounce Rate
In Web analytics, including Google Analytics, bounce rate is the measurement in percentage of how many Web site visitors view only one page within your Web site – this is the one page they entered the site on (called the entrance page or landing page). These visitors view only that single page and exit the site on that same page. A high bounce rate typically indicates that the landing page isn't relevant to your visitor or you are not offering any incentive for the visitor to explore other areas and pages of your website.
Google recommends that Web site publishers lower bounce rates by tailoring landing pages to each keyword and ad that you run so landing pages consistently provide the information and services that were "promised" in the ad copy.
2) Email Marketing Bounce Rate
In email marketing bounce rate refers to the percentage of email addresses in your subscriber list that didn’t receive your message because it was returned by a recipient mail server. There are two categories of email bounces including hard bounce and soft bounce.
See also the "Small Business Marketing Guide" on Webopedia.com for more information on email marketing.