Fully asynchronous version of the AdSense ad code.
https://support.google.com/adsense/answer/3221666?hl=en
Advertising
Get access to data on 5,304,455 websites that are Google Adsense Asynchronous Customers. We know of 2,300,346 live websites using Google Adsense Asynchronous and an additional 3,004,109 sites that used Google Adsense Asynchronous historically and 1,360,143 websites in the United States.
Get a list of 5,304,455 websites using Google Adsense Asynchronous which includes location information, hosting data, contact details, 2,300,346 currently live websites and an additional 581,089 domains that redirect to sites in this list. 3,004,109 sites that used this technology previously and 1,360,143 websites in the United States currently using Google Adsense Asynchronous.