See: Unique Visitor
A visitor to a website or Internet resource is at a high level simply an individual who has chosen to direct their browser (or other tool) at that website or Internet resource. They are more often identified as an IP that has visited the site. This is to be differentiated from a unique visitor which would aim (albeing not perfectly) to avoid double counting of visits in a better attempt to identify how many actual individuals have visited the site during a specified period.