A person who is subject to, liable for, or pays tax as opposed to a nontaxpayer who is neither the subject nor the object of revenue laws.
All of the people, collectively, in a population who pay tax (especially used in the context of the government financing something using the tax revenue).
1945: I got into the bus full of taxpayers who were giving some money to a taxpayer who had on his taxpayer's stomach a little box which allowed the other taxpayers to continue their taxpayers' journeys. I noticed in this bus a taxpayer with a long taxpayer's neck and whose taxpayer's head bore a taxpayer's felt hat encircled by a plait the like of which no taxpayer ever wore before. Suddenly the said taxpayer peremptorily addressed a nearby taxpayer, complaining bitterly that he was purposely treading on his taxpayer's toes every time other taxpayers got on or off the taxpayers' bus. Then the angry taxpayer went and sat down in a seat for taxpayers which another taxpayer had just vacated. — Raymond Queneau, 'from Polyptotes' in Exercises in Style, 1945