How is a Unique Visitor counted in Piwik?
Unique Visitors is the number of visitors coming to your website; Unique Visitors are determined using cookies. If the visitor doesn't accept cookie, a simple heuristic is used to try to match the visitor to a previous visitor with the same features (IP, resolution, browser, plugins, OS, ...).
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).
Unique Visitor 是通过cookie判断的，若cookie禁用，则通过IP,分辨率，浏览器，插件，操作系统等因素与前一访问者比较进行判断。
同一 Visitor 间隔 30分钟内的访问只算一次visit。 可配置