John,
The bounce rate is calculated on a single page visit. Or at least that is the way that it has always been in the past. If you think about it, it makes sense. At the time the stat was started it was to attempt to measure the accuracy of the search engine algorithms. After all, Goggle wrote and invented the stat and they were trying to improve their search algorithms.
The belief was that is you performed a search for a particular topic and found that topical search was accurate you visited and moved around the site with the successful search.
If you immediately left the site the topical search was believed to be inaccurate and that was the reason that you left. Keep in mind that this was quite a while ago that this stat was started and that statistical factor has evolved tremendously since the original inception. In the old days there were not 10 of millions of articles on a single topic and the web had not grown to a MEGA-STORE environment. Where you have content based solely appearance.
Now there are many more factors to consider when looking at a bounce rate because of all the competition in the same topical space. Now not only do you need to have great content, it must also be attractive, attention getting, load quickly, be different and accurate.
But getting back to your main question, according to the guys that wrote the original algorithm, it is the percentage of users that navigate off the website from the page they landed on without opening any additional site pages or links.