Fully asynchronous version of the AdSense ad code.
https://support.google.com/adsense/answer/3221666?hl=en
Advertising
Get access to data on 5,133,340 websites that are Google Adsense Asynchronous Customers. We know of 1,862,223 live websites using Google Adsense Asynchronous and an additional 3,271,117 sites that used Google Adsense Asynchronous historically and 997,750 websites in the United States.
Get a list of 5,133,340 websites using Google Adsense Asynchronous which includes location information, hosting data, contact details, 1,862,223 currently live websites and an additional 905,854 domains that redirect to sites in this list. 3,271,117 sites that used this technology previously and 997,750 websites in the United States currently using Google Adsense Asynchronous.