The word "citizen" is inseparable from the word "constitution." Only when a government is constitutional, only in a republic, are there some individuals who are citizens. It may be that only some of the people are citizens. It may be that only some of the people are governed with their consent and with political liberty; or it may be that, except for certain justifiable disfranchisements, all human beings constitute the people who are governed with their consent, and who have political liberty.
First, there are those who are governed as slaves, governed without their consent, without participation in government, and for the good of their masters, not for their own good.
Second, there are subjects; individuals who are governed despotically because they are thought incapable of participating in government. Those who are infants or below the age of consent are governed despotically, but the despotism is benevolent if the parents are concerned with the welfare of the progeny they rule. Adults of a conquered people are despotically governed as subjects, sometimes benevolently, sometimes tyrannically, for the good of the ruler rather than for the good of the subjects.
In various epochs and places before the liberation of women, female adults in oligarchies with restricted suffrage were governed despotically as subjects. The first step toward their emancipation was granting them suffrage. They then became citizens.
Universal suffrage makes citizens of all who are above the age of consent and participate in their own government. That gives us the definition of citizenship. It belongs to those who are given political liberty and equality, those who are governed with their own consent and have a voice in their own government. All citizens are politically free. They are not all politically equal, however, for in constitutional governments citizens maybe temporarily or permanently in public office. When that is the case, they have more political power than those who are not in public office, because they have more political functions to discharge.
Another way of defining citizenship is to speak of citizens as having a share in sovereignty. As citizens, all have an equal share.
The existence of citizenship depends upon the existence of constitutional government, but that government need not be democratic, as the government of Athens was not democratic in the fifth century B.C., and the government of the United States was not democratic in the eighteenth and nineteenth centuries. Both were oligarchies with unjustly restricted suffrage. The populations included slaves, and subjects as well as citizens; those who were citizens consisted of a minority of the population. Democracy comes into existence only with universal suffrage, which gives citizenship to all who by natural right are entitled to it. (See Constitution and Democracy.)