Google defines it as "the percentage of single-page visits or visits in which the person left your site from the entrance (landing) page." It's basically the percentage of visitors who enter the site and "bounce" (leave the site) rather than continue viewing other pages within the same site. The question I have always had is the length of time they have to stay and leave that single page before they are considered a "bounce." I'm some of the experts here can help us both out.
