A unique visitor is a term that is often heard in web analytics. A unique visitor is someone who accesses a website once within a period of time. However, this visitor is not included in the number of accesses if they access it again many times. It only counts as one visitor to the site.
This may seem unfair, but in the web analytics world this allows one person to not get counted many times because they click over and over again on the same site in order to show that there is a lot of traffic coming onto the site. However, there is a problem called the hotel problem in web analytic which refers to these unique visitors numbers not adding up correctly.