An ad impression is reported whenever an individual ad has begun to download to the user’s device. Different ad formats will display varying numbers of ads; for example, each time a vertical banner appears on your site, you'll see two ad impressions in your reports. In addition, keep in mind that the number of ads in any ad unit may vary depending on whether the ad unit is displaying standard text ads expanded text ads, or display ads.
Note that this metric is only available under the "Custom" metric family tab. Learn more about the custom metric family.