Definitions for "Unique Visitors"
The number of individual users who visit the site. Based on requests by unique IP addresses.
The number of visitors who visit a web site within a given period of time (say 24 hours) from a single IP address. If a person visits the site three times a day, it is counted as one visit for that day.
A count of individual users who have accessed your web site. It should be noted that the "user session" metric does not yield an accurate unique visitor count, as multiple user sessions can be generated by one unique visitor.