Definitions for "Ad Impression"
1) An ad which is served to a user's browser. Ads can be requested by the user's browser (referred to as pulled ads) or they can be pushed, such as E-mailed ads; 2) A measurement of responses from an ad delivery system to an ad request from the user's browser, which is filtered from robotic activity and is recorded at a point as late as possible in the process of delivery of the creative material to the user's browser -- therefore closest to the actual opportunity to see by the user.
the act of sending an ad to a user's browser.
The number of times an ad banner is displayed on a website.