A unique user is an identified visitor on your web site. They are given a tag with information about their IP address, browser ID or other identification. A visitor is marked as a unique user for a specific period of time, e.g. a week or a month.
In order to become a unique user, the person has to visit a web site and receive some content. It can be ads, newsletters, e-mails.
From Starting Your Own Affiliate ProgramGet Started