Everybody who has used Google Analytics will have seen statistics about their 'bounce rates.' I have often heard people say that if a visitor clicks away from your website in under 5 seconds, then it registers as a bounce. But this is not true.

This is an interesting discussion about what Google really considers to be a bounce.

According to Avinash Kaushik from the Google Analytics team, a bounce is

a visit (a session technically speaking) with just one page view.

So Avinash is saying that it doesn't matter how long the visit is, if only one page is viewed then it is considered a bounce. The only exceptions mentioned are if you use Google Analytics and 'event tracking' for your videos, or you have a 100% flash site.

You can read a more in-depth piece about bounces and bounce rates here