Individuals who have visited a Web site (or network) at least once in a fixed time frame, typically a 30 day period.
Most measurements of unique visitors are estimates.
Sites often calculate unique visitors based on the IP address information found in the log files, and sometimes through cookies. However, many factors may skew the results.
Traffic rating companies typically calculate unique visitors by monitoring actual usage of a group of volunteers, then applying the results to to the Internet population. Results fluctuate considerably for small sites due to their small sample sizes.
Lies, Damned Lies, and Unique Visitors
E-Commerce Times (June 21, 2000)