Its a measurement of responses from an ad delivery system, to an ad requested by the user's browser, which is recorded in a certain format depending on the delivery system. Web pages have ads. Once the visitor has viewed an ad on a web page an impression is recorded. Ad Views are different from Page Views.
With the objective of knowing that there was an opportunity to see an ad, Audit Bureau of Verification Services defines an ad impression as an advertisement that was delivered to a qualified visitor. We define it as the delivery of an online advertisement (linked banner or button), to a page of information requested by a Salt Lake Tribune or Deseret News online edition visitor.
A request to the server to return an ad to the appropriate web page. The IAB issued proposed standards using ad requests as the unit of measure for counting web ads. Also referred to as Ad Requests.