Citizen: 1. A legally recognized subject ornational of a state or commonwealth, either native or naturalized. 2. An inhabitant of a particular town or city.
Denizen: 1. An inhabitant or occupant of a particular place.
I would say the answer depends on how technical we're being.
A citizen of the United States is a legal resident who has been processed by the government as being a member of the United States.
A denizen of the United States is simply someone that lives there.
Technically speaking, one could never be, for example, a citizen of the Earth -- but we're all denizens of the Earth.
I can think of three types of use where denizen works, and citizen doesn't.
"Citizen" means a person who is not just present in a city or other conurbation, but at least potential part of its social body.
"Denizen" is a much more general word which is not limited to humans, and not limited to civilised or organised places. It often has a connotation of wildness.
This is fine distinction, and may have a lot to do with what time frame one is working in, and the legal ramifications of the term. a monarch could confer denizenship on a foreign person, with all the rights of natural born citizen, but the monarch could also revoke it. Someone who was naturalized was considered a natural born citizen with all the privileges that entailed. A fine hair to split, to be sure.
"Denizen" is preferable to a sentient nonpartisan observer in self-description. The legal term "citizen" makes the claimant liable for the actions of corporate Person/actors.
I disagree with this answer:
'A citizen of the United States is a legal resident who has been processed by the government as being a member of the United States.'
A denizen is simply one who resides in a location.
A subject is one who has been declared a member of a nation by a Ruler or State.
A citizen is one who has declared himself a member of a nation.
Citizenship arises from the individual alone. A state cannot grant or deny citizenship. Only an individual can do this.
Edit Jan 31,2018
I double down:
A CITIZEN is a FREE MAN;
one who has CHOSEN;
CONSENTED to support a GOVERNMENT.
One who has a natural law OBLIGATION
to RENOUNCE support and obedience
to any social contract, with said institution,
which denies the INALIENABLE RIGHTS of another FREE MAN.
The Social Contract by Jean-Jacques Rousseau:
“Every man having been born free and master of himself, no one else may under any pretext whatever subject him without his CONSENT.
Second Treatise of Government by John Locke:
"Men being, as has been said, by nature, all free, equal and independent, no one can be put out of this estate, and subjected to the political power of another, without his own CONSENT."