Fully asynchronous version of the AdSense ad code.
https://support.google.com/adsense/answer/3221666?hl=en
Advertising
Get access to data on 4,645,874 websites that are Google Adsense Asynchronous Customers. We know of 1,883,966 live websites using Google Adsense Asynchronous and an additional 2,761,908 sites that used Google Adsense Asynchronous historically and 1,020,932 websites in the United States.
Get a list of 4,645,874 websites using Google Adsense Asynchronous which includes location information, hosting data, contact details, 1,883,966 currently live websites and an additional 901,830 domains that redirect to sites in this list. 2,761,908 sites that used this technology previously and 1,020,932 websites in the United States currently using Google Adsense Asynchronous.