Unique user
According to IFABC Global Web Standards, a unique user (UU) is "An IP address plus a further identifier. The term "unique visitor" may be used instead of "unique user" but both terms have essentially the same meaning (see below). Sites may use User Agent, Cookie and/or Registration ID." Note that where users are allocated IP addresses dynamically (for example by dial-up Internet service providers), this definition may overstate or understate the real number of individual users concerned. Unique users is a common way of measuring the popularity of a website and is often quoted to potential advertisers  or investors. A website's unique users are usually measured over a standard period of time, typically a month. Use of performance indicators such as unique visitors/users is controversial, with Greg Harmon of Belden Research inferring that many companies reporting their online performance "may overstate" the number of unique visitors. Remember, it's just an identifier of a computer, not a person. And usually, the computer is identified by a "cookie" which is most often specific to an individual browser on that computer. Since an increasing percentage of people in the United States (at least) now have access to  a computer at home and at work or school, one may have to divide the reported total of unique users in half. Then, another increasing fraction of people regularly delete cookies from their machines—presumably both at home and at work—and yet another large fraction use more than one browser on each of their machines. This means that for a typical news site, for example, which people might typically visit more than once a day to keep up with breaking news, the reported unique users might overstate the number of different people by a factor of four. On the  plus side, for those wishing to impress advertisers or investors, the reported number of sessions or visits and pageviews are probably more accurate, so that smaller group of people visits much more often and looks at more pages than the raw numbers would suggest. Understanding unique users numbers. Similar to the TURF (Total Unduplicated Reach and Frequency) metric often used in television, radio and newspaper analyses, Unique Users is a measure of the distribution of content to a number of distinct consumers. A common mistake in using Unique User numbers is adding up Unique User numbers across dimensions. A Unique  User metric is only valid for its given set of dimensions e.g. time, browsers. For example, a website may have 100 unique users on each day (day being the dimension) of a particular week. With only this data, one cannot extrapolate the number of weekly Unique Users (only that the Unique User count for the week is between 100 and 700). However, website administrators who can track unique user traffic over a longer period of time can build up a reliable view on their performance against direct competitors within the sector. Online businesses tend to have a static conversation  rate ratio between unique users and new business clients. When calculating movement of unique users through conversion funnel the same time period must be used at every step. Limitations of unique user numbers. Unique user counts for websites are typically counted by using cookies. When a browser visits a website, the website checks for the existence of a particular cookie. If the cookie is present, the cookie value is captured. If the cookie is not present, the website will create a cookie. Unique visitor. Unique visitors refers to the number of distinct individuals requesting pages from the website during a  given period, regardless of how often they visit. Visits refer to the number of times a site is visited, no matter how many visitors make up those visits. 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 may 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.