noun

• (Christianity) A resident of the heavenly city or (later) of the kingdom of God: a Christian; a good Christian.
• (obsolete) An ordinary person, as opposed to nobles and landed gentry on one side and peasants, craftsmen, and laborers on the other.
  "[W]ould Mr. Delvile, who hardly ever spoke but to the high-born, without seeming to think his dignity somewhat injured, deign to receive for a daughter in law the child of a citizen and tradesman?"
• (now historical, usually capitalized) A term of address among supporters of the French Revolution in France or elsewhere; (later, dated) a term of address among socialists and communists.
  "Citizen, I desire nothing more than to get to Paris, though I could dispense with the escort."
• (computing, figurative) A notional inhabitant of a software system; an object or a software application.
  "The HIG delivers Apple's design commandments, the company's definition of what it means to be a good iPhone citizen."