Impression sometimes called a view or an ad view, is a term that refers to the point in which an ad is viewed once by a visitor, or displayed once on a web page.
The number of impressions of a particular advertisement is determined by the number of times the particular page is located and loaded. If it is randomly generated, then it is the number of times the particular ad appears from the random generator.
An impression is an estimate of the number of people a particular advertisement is reaching, and may be counted in different ways depending upon the way the ad is situated on the page, as well as the number of times the web page where the ad appears is shown.
Ad reports list the number of total times of impression of the ad, which basically counts the number of times that the ad was served by the search engine when the keywords that the ad is connected to were searched for.