A term used in web analytics to describe a person who visits your website one or more times within a certain period.

The visitor is considered "unique" because s/he is only counted once during the reported period -- assuming that person is using the same device/IP address to access your site.

Because the visitor is counted only once -- despite the number of times that person visits your site -- you're able to get a more accurate measure of the number of people who are coming to your site.