A citizen is a legally recognized member of a political or civil community. In general, citizens may be native (born in the community) or naturalized (immigrants).
In ancient Greek city states citizens formed a social class contrasted with helots,slaves, non-resident aliens and others. In the U.S. and other modern political communities aspiring to universal suffrage, or full adult citizenship, the category of non-citizen residents is often very small and most adult men and women are regarded as citizens.
The term need not be limited to recognized members of nation states. Thus, registered and named contributors to Citizendium are, indeed, citizens.