First Citizen was a title which existed in the Roman Empire-like state on planet 892-IV.
One holder of this title was Captain R.M. Merik. Merik had arrived with his ship, the SS Beagle on this planet in 2262. While many of his crew were subsequently put to death in the arena, Merik, now called Merikus, became First Citizen and also Lord of the Games.
The title is roughly equivalent to the Roman Empire’s title of “Princeps” or first citizen. This title was held by the ancient emperors of the Roman Empire on Earth. Because of 892-IV parallel development to Rome, it is likely that the First Citizen is the “emperor.” Despite this, it is clear that true power can be held by others, as indicated by Merik’s puppet status to Proconsul Claudius Marcus. (TOS: "Bread and Circuses")