Very interesting and valuable domain. At first look it seems advert-related however the "AD" stands for hundreds of two-word combinations, especially military, health, business, tourism and sport. Suggest you check out the Abbreviations website and run a search for "AD" abbreviations and acronyms. Always run a WIPO trademark search, but a great thing about this domain is it's so generic I don't think you'd have legal problems.
Research which of these has a magazine/publication and what domain they are using: