Aristotle asked what being a citizen means. For him the citizen was a particular thing that not everybody was. A citizen might be called on to serve in office and to make decisions for the state. We might consider jury duty and voting as along that line, but there are a lot of 'citizens' who don't vote, and registering to vote as something that might result in being called to serve on a jury. He counted children as partial citizens to the degree that they would become full citizens when they were old enough. By that criterion, how many of our 300 million would be counted as citizens?
Did you ever read the book or see the movie, "Starship Troopers"? They had the right idea about citizenship.