National Guard: A state-level military reserve composed of citizen-soldiers who can be called upon by their respective states or federal government during emergencies.
Well-regulated Militia: Often mentioned in discussions about the Second Amendment, it refers to militias that are properly trained and organized under some form of regulation.
Posse Comitatus: A legal principle that limits the powers of federal military forces from enforcing domestic laws within U.S. territory unless authorized by Congress.