Designing a game is, in essence, creating a system in which one or more players interact with the system by making interesting choices which influence the system - resulting in success or failure.
This holds true whether the game is an economic game (more like a ThoughtExperiment), a board game (Monopoly), a card game (Bridge), or a ComputerGame (The Sims). The format of the game influences the definition of interact and set limits on the scope of the system.