My dictionary offers "A resident of an area represented by an elected official".
This is actually acutely important to both the history of the United States of America and to a grasp of how a representative democracy (if the US is to remain one) works
So, let's go back to the mid-18th century. America is a colony. The colonists are unhappy because they feel unrepresented, but in Westminster, London, the Members of Parliament are content that indeed those colonists are represented - by them - even though the colonists don't get a vote. This concept is named "virtual representation" and we'll return to it. The colonists oppose it, "No taxation without representation" they cry.
It is worth taking a moment here to consider that those Members were not elected by the majority of the people in the districts they were elected to. Only wealthy men in England could vote at that time, a tiny fraction of the population who'd be affected by the Parliament's decisions. This is also prior to Great Reform, so the districts represented do not closely resemble one another in population (though next century things would get much worse before they got better)
So, the Americans revolt, and having won their independence they set up for themselves a new system of government. In many ways this system is reminiscent of the one they've just got rid of, except that (no surprises) the rich old white men who didn't like the old system are instead placed in charge of the new one. In particular, the United States of America does not in fact enfranchise all its citizens, it actually only gives the vote to wealthy men, almost exactly the same as England had done. Still, they now feel represented (and indeed have become important political figures in their new country) and so for them it seems mission accomplished.
The US Congress offers exactly the same weak rationale as the English parliament, virtual representation. Sure, women, the poor, slaves, children and so on are not allowed to vote but we promise your Congress people are representing you anyway.
And so that's the status quo. Either you aren't really a representative democracy, with your millions who are disenfranchised for various reasons†, or you accept that representation is necessarily distinct from enfranchisement - the right to vote.
† In both England and the US the franchise was greatly expanded, to include more men, and eventually women, but although such expansions are almost invariably described as "universal" by their proponents in both countries many people can't vote, and neither is close to even achieving the more reasonable standard of allowing everybody who can meaningfully express an opinion (which would rule out newborns for example) the vote. Not least both prevent people from voting if they are in prison, as if somehow prison reform wouldn't be a very significant issue those people might now have more insight into than other voters...