A real person who has visited your website, not a crawler/spider/bot. No matter how much time, and how many links they follow, it is only a single unique visit. They are usually tracked by their IP address when on the site.

Uniform Resource Locator (or URL) is the location of a resource on the Internet. By that we mean the location of a Web page or website. For our convenience, this is done primarily through the use of domain names, as it is easier for humans to remember words, rather than numbers (like 192.168.131.65).