Here is an exmple of impressions as well. I had to do some research on this myself.
An ad impression, also known as an ad view, is a single instance of a single advertisement appearance. One visitor can view many ads. A raw/gross impression is counted every time the ad is displayed to a visitor. It includes duplications and repeated viewings. For example: if a web page has 4 ads, 4 gross impressions will be recorded, one for each ad.
On the other hand, only one unique impression is counted per 24-hour period, even when the visitors view the same ad multiple times. After 24 hours, if this visitor views this ad again, another unique impression is counted. The reports for unique impressions/IPs show the activities of your visitors. Accurate unique impression counting relies on information from the cookie stored on your visitors' browser. When the cookie is removed/deleted or disabled, a unique impression is the same as a gross/raw impression. If a visitor clears his/her cookie frequently, the unique counter will also increase.
Visitor A viewed: Ad1, Ad2, Ad3, Ad2
Visitor B viewed: Ad1, Ad4
Ad1 has 2 unique and 2 gross impressions
Ad2 has 1 unique and 2 gross impressions
Ad3 and Ad4 each has 1 unique and 1 gross impression
The number of unique impressions is always smaller than the number of gross impressions because one visitor usually sees more than one ad.
Other terms include "ad exposure", "ad view" or simply "imp". A gross impression is the default term and is simply referred to as an impression.
Our unique impression number is defined as per visitor (IP address) per ad within 24 hours. If another ad server counts a unique impression per 12 hours, the unique numbers will be different across the two reports.
Impressions vs. Visits
A visit is often defined as a single session from a visitor. A visitor can have multiple visits. A visit can generate multiple page views. If a page has multiple ad placements, the number of gross impressions will be many times higher than the number of visits. For example: a visitor visited your website twice on the same day. During his first visit, he viewed 2 pages, and saw 4 ads. During his second visit, he viewed 3 pages, and saw 6 ads. The number of gross impressions in this case is 10 impressions (4 plus 6).
Impressions vs. Pageviews
Ad impressions are not the same as pageviews. For example, when a visitor views a page with 4 ads, there is one page view but there are 4 ad impressions. On the other hand, if a page does not have any ad, that is one page view but no ad impression.
Unique Visitors vs. Unique Impressions
For example, a visitor can view 20 pages. This will record as one unique visitor by the website log/analytics program. However, if this visitor sees 5 different ads during those 20 pages, the same visitor records 5 unique impressions, one unique impression for each ad.