AdSense Ad Units Viewed is a Metric in Google Analytics under the Adsense section
AdSense Ad Units Viewed Definition: The number of AdSense ad units viewed (requires integration with AdSense). An ad unit is a set of ads displayed as a result of one piece of the AdSense ad code. For details, see https://support.google.com/adsense/answer/32715?hl=en.
Data Analytics & Insights Google Analytics Metrics AdSense Ad Units Viewed