Real Dictionary

National Guard
countable noun: In the United States, the National Guard is a military force within an individual state, which can become part of the national army if there is a war or emergency.

National Guard in American English
in the U.S., the organized militia forces of the individual states, a component of the Army of the U.S. when called into active federal service
noun: state military forces, in part equipped, trained, and quartered by the U.S. government, and paid by the U.S. government, that become an active component of the army when called into federal service by the president in civil emergencies

National Guard in British English
noun: the armed force, first commanded by Lafayette, that was established in France in 1789 and existed intermittently until 1871