Fully asynchronous version of the AdSense ad code.
https://support.google.com/adsense/answer/3221666?hl=en
Advertising
Get access to data on 5,714,752 websites that are Google Adsense Asynchronous Customers. We know of 1,969,220 live websites using Google Adsense Asynchronous and an additional 3,745,532 sites that used Google Adsense Asynchronous historically and 1,042,646 websites in the United States.
Get a list of 5,714,752 websites using Google Adsense Asynchronous which includes location information, hosting data, contact details, 1,969,220 currently live websites and an additional 970,757 domains that redirect to sites in this list. 3,745,532 sites that used this technology previously and 1,042,646 websites in the United States currently using Google Adsense Asynchronous.