Citizen, subject, national are comparable when denoting a person who is regarded as a member of a sovereign state, entitled to its protection, and subject to its laws.
Citizen implies alien as its opposite. It is applicable to a native or naturalized person, regardless of sex or age, who owes allegiance to a government and is entitled to its protection of his life, liberty, and property at home or abroad.
Ordinarily (as in the United States) citizenship does not imply possession of all political rights (as the right to vote).
Citizen often implies allegiance to a government in which the sovereign power (theoretically or absolutely) is retained by the people; it is usually the preferred term in designating those persons in a republic whose status is not that of aliens.
Subject is applicable to a person, no matter where he resides, who by right of birth or naturalization owes allegiance to a personal sovereign (as a king or emperor) whether this sovereign rules directly or is a figurehead in whose name an often representative government is conducted; thus, subject is the preferred term in the British Commonwealth of Nations largely for historical reasons in spite of the limitations on the power of the sovereign and in spite of the representative form of government in Great Britain and in its dominions.
The term is also applicable to any person residing in territory governed by another state that has gained power over it by force of arms or by conquest, whether the sovereign power in that state is vested in a person or in the people.
National belongs with this group of terms in spite of its shifting significance and more or less conflicting implications. It is applicable chiefly to any of a body of persons of the same nation or people living in a country other than the one in which they have or have had the status of citizen or subject. In diplomatic use the term is often applied to one’s fellow countrymen.
Still other denotations, not so common, have come into use. Chief among these is the definition of a national as anyone who has been born in the territory of a given government, even though he now resides in another country, either as an alien or, by naturalization, as a citizen or subject of that country.
There is also a tendency to prefer national to subject or citizen in some countries where the sovereign power is not clearly vested in a monarch or ruler or in the people or where theories of racism prevail. In some use, especially in international law, national is applied to anyone entitled to the protection of a government regardless of whether his status is that of citizen or not; in this sense, the Filipinos were formerly nationals, though never citizens, of the United States.