unique users, unique visitors, uniques, unique impressions

Unique users refers the the number of distinct individuals, as determined by IP address, user login, cookie, or some combination thereof, who visit a web site or view a banner ad. Most ad networks favor sites which generate a large number of unique impressions over sites whose users view large numbers of pages.

A typical ad network will often not show more than five or six individual ads of any one type to a single user in a single browser session. Ad networks for larger sites, such as Tribal Fusion, require sites to be visited by at least 5,000 unique visitors per day. Networks such as Burst and Fastclick require around 3,000 unique visitors per month.