Unique visitors (or visitors) refers to the number of distinct individuals requesting pages from the website during a given period, regardless of how often they visit. (Visits refers to the number of times a site is visited, no matter how many unique visitors make up those sessions.) When an individual goes to a website on Tuesday, then again on Wednesday, this is recorded as two visits from one visitor.
The purpose of tracking unique visitors is to help marketers understand website user behavior. Because a visitor can make multiple visits in a specified period, the number of visits will be greater than the number of visitors. A visitor is sometimes referred to as a unique visitor or a unique user to clearly convey the idea that each visitor is only counted once.
The measurement of users or visitors requires a standard time period and can be distorted by automatic activity (such as bots) that classify web content. Estimation of visitors, visits, and other traffic statistics are usually filtered to remove this type of activity by eliminating known IP addresses for bots, by requiring registration or cookies, or by using panel data.
See also 
- Farris, Paul W.; Neil T. Bendle; Phillip E. Pfeifer; David J. Reibstein (2010). Marketing Metrics: The Definitive Guide to Measuring Marketing Performance. Upper Saddle River, New Jersey: Pearson Education, Inc. ISBN 0-13-705829-2. The Marketing Accountability Standards Board (MASB) endorses the definitions, purposes, and constructs of classes of measures that appear in Marketing Metrics as part of its ongoing Common Language: Marketing Activities and Metrics Project.