What makes a domain, besides money? A domain is an area of military operations characterized by unique physical properties that require specialized forces with their own way of operating. Just as the air requires planes, the sea requires ships, and space requires satellites and rockets, the electromagnetic spectrum requires radios, radars, antennas, and jammers. Domains are physically distinct, but they can overlap each other. Just as air and airpower extend over both land and sea, and space overtops them all, cyberspace exists wherever electronic circuits do, and electromagnetic radiation goes everywhere.