The number of unduplicated visitors coming to your website. Every user is only counted once, even if he visits the website multiple times a day.
Unique visitors are determined using first party cookies stored in your visitor browser. When a same person visits your website on two different devices (for example their laptop and on their mobile phone) then Piwik will detect two unique visitors.
If the visitor’s browser does not accept the Piwik cookie (if they have disabled, blocked or deleted cookies), a simple rule of thumb is used to try to match the visitor to a previous visitor with the same features (IP, resolution, browser, plugins, OS, etc). Read about all details in this other FAQ.
Note that by default, unique visitors are available for days, weeks and months periods, but unique visits are not processed for the “year” period for performance reasons. Learn how to enable unique visitors for all date ranges.
See also our glossary of analytics terms.