Nomic is a game whose rules of nomic are not written in stone. In fact, the object of the game is to make changes to the rules of the game. Players start off following some "initial rule set", which dictates how the rules can be changed. Once a rule change has been made, players then follow this new rule set. Most importantly, the rules about how rule changes are made can themselves be changed!
This is where the core of the game lies, because as a result of these rule changes, the game you are playing will change from moment to moment. The nature of the rule changing mechanism might change from democratic to capitalist, to totalitarian, to whatever. Or the ability to change the rules might be removed entirely - perhaps the game will turn into chess, or tag, or snap. The future of the game is entirely in the hands of the players.
In the words of Nomic's author, Peter Suber: "Nomic is a game in which changing the rules is a move. In that respect it differs from almost every other game. The primary activity of Nomic is proposing changes in the rules, debating the wisdom of changing them in that way, voting on the changes, deciding what can and cannot be done afterwards, and doing it. Even this core of the game, of course, can be changed."
Most Nomic enthusiasts seem to enjoy playing the game in order to experience the possibilities of different kinds of lawmaking processes, and also to exercise their ingenuity in trying to discover (or create) loopholes in the rules which give unusual results - mostly to the benefit of the player.
Nomic was conceived and designed by Peter Suber, and first published in Douglas Hofstadter's column "Metamagical Themas" in Scientific American in 1982, and later in Hofstadter's book of the same name. Peter revised the rules and published them in his own book, "The Paradox Of Self Amendment" in 1990.