An impression is counted when one or more pixels of the ad creative is visible on a device's screen.
Note: When a third-party waterfall ad source serves a banner ad, AdMob counts an impression when the third-party adapter sends the ad load callback. This may cause discrepancies in reported impressions between AdMob and your third-party ad source. For example, if you preload banner ads that aren’t shown immediately, you may notice higher impressions reported in AdMob.