How is a visit defined in Piwik?
If a visitor comes to your website for the first time, or if he comes returns to your website 30 minutes after his last page view, this will be recorded as a new visit. You can change this value for measuring visits (session timeout) in the configuration file (config/global.ini.php) by adjusting the visit_standard_length value (defaults to 1800 seconds, 30 minutes).
If a visitor comes to your website for the first time, or if he visits a page more than 30 minutes after his last page view, this will be recorded as a new visit. A Unique Visitor can make multiple Visits per day. You can change the 30 minutes value (session timeout) by editing in the configuration file (config/global.ini.php) the visit_standard_length value (defaults to 1800 seconds, 30 minutes).