Homeland security is an American umbrella term for "the national effort to ensure a homeland that is safe, secure, and resilient against terrorism and other hazards where American interests, aspirations, and ways of life can thrive to the national effort to prevent terrorist attacks within the United States, reduce the vulnerability of the U.S. to terrorism, and minimize the damage from attacks that do occur."
Homeland security is not constrained to terrorist incidents. Terrorism is one of many threats that endanger society. Within the U.S., an all-hazards approach exists regarding homeland security endeavors. In this sense, homeland security encompasses both natural disasters and man-made events. Thus, the domain of homeland security must accommodate a plethora of situations and scenarios, ranging from natural disasters (e.g., Hurricane Katrina) to acts of terrorism (e.g., Boston Marathon bombing).
The term came about following enactment of the Homeland Security Act of 2002 and reorganization of many U.S. government civil agencies effective March 1, 2003, to form the United States Department of Homeland Security after the September 11 attacks, and may be used to refer to the actions of that department, the United States Senate Committee on Homeland Security and Governmental Affairs, or the United States House of Representatives Committee on Homeland Security.
Homeland defense (HD) is the military protection of U.S. territory, sovereignty, domestic population, and critical infrastructure against external threats and aggression.