ground rule
A basic guideline or governing principle of procedural conduct in a given situation or endeavor. Often used in the plural ("ground rules"). The phrase is a reference to baseball, in which different ballparks have specific ground rules that govern play for that particular field. Before we begin the debate, let's establish some ground rules about what may and may not be discussed OK, class, here are the ground rules: no talking during class and no cell phones. Follow those and you'll stay on my good side.