How does google see the bounce rate?!?
That's a really good question. And in fact, there are actually two (at least) definitions of bounce rate, based on how Google sees it.
One, of course, is if you are using Google Analytics. But it's also possible they could use info from Adsense, or perhaps some other kind of readable footprint, depending on what's on your site.
So in Google Analytics, it will show your bounce rate and this is based on the number of visitors who come to your site and only view one page. Certainly, Google can not measure this for every website. Plus, it's not actually a particularly useful metric.
But in his paper, Panda's definition of bounce rate is different. It's the number of visitors who go to your site and "immediately" return to the search engine. He doesn't specifiy exactly how long immediately is.
But I think this is the type of "bounce" that they are probably looking for in terms of ranking. It just makes sense.
There's all this talk about "quality". But Google can't measure quality directly. They have to use "signals" - measurable variables that can tell you something about the quality of a page. Well, if someone comes to your page and stays for 2 seconds and then goes right back to Google, that's a pretty good indication that your page sucks.
So I suspect that's how Google determines bounce rate -- immediate returns to the search engine.
BTW, if this is the case, that means that the only bounces that matter would be from Google. Depending on your traffic methods, this could be very important, as nearly 100% of traffic from places like Digg and Stumbleupon are "bounces" if you use the 1st method (visiting multiple pages) but are NOT bounces using the second, more likely, method of immeidate returns to Google. In this definition, only search engine traffic can be a bounce.