It's the number of distinct "3rd-party" domain names reached on average for the 15 visited web pages. So in the case of Disconnect, for the 15 URL visited, on a

It's the number of distinct "3rd-party" domain names reached on average for the 15 visited web pages. So in the case of Disconnect, for the 15 URL visited, on average 93 "3rd-party" domains were touched. The web page domain in the URL address bar is deemed 1st-party. "Domain" is extracted as per Mozilla's Public Suffix List.