What is an ad impression?
An ad impression is counted whenever an ad is displayed within an app.
Impressions equal viewable impressions. Ad impressions are counted by each network directly when the ad unit was successfully opened and is displayed on the screen. Once this happens, the networks report the impression to the mediation layer and an impression is counted. In general the networks are reporting impressions when there is at least 1 pixel of the ad in view for at least 1 second for display ads, and 2 seconds for video ads.
Ad impressions vs clicks should not be confused with each other. In other words, impressions in digital marketing tells advertisers and developers how many times an ad was viewed by users. The ad impressions definition is not contingent how many times the ad was clicked.
App developers get paid for every 1,000 ad impressions, which is measured by eCPM.