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unique visitorsA count of how many different people access a website. For example, if a user leaves and comes back to the site five times during the measurement period, that person is counted as one unique visitor, but would count as five "user sessions."
Unique visitors are determined by the number of unique IP addresses on incoming requests that a site receives, but this can never be 100% accurate. Depending on configuration issues and type of ISP service, in some cases, one IP address can represent many users; in other cases, several IP addresses can be from the same user.