For every visit on your website Google Analytics classify the visitor as "First Time Visitor" or "Prior Visitor". If any visitor have ever visited your website before the date range selected, then it is counted as prior visit, however if the visitor is visiting your website first time in selected date range then it is an Absolute Unique Visitor. For more info on Absolute Unique Visitor Refer to: http://analytics.blogspot.com/2007/01/absolute-unique-visitors-versus-new.html