Английский словарь американских идиом - ground rule

{n.} 1. A rule in sports that is made especially for the grounds or place where a game is played. - Usually used in the plural. * /There was such a big crowd at the baseball game, that the ground rules of the field were changed in case a ball went into the crowd./ 2. A rule, usually not written, of what to do or how to act in case certain things happen. - Usually used in the plural. * /When you go to a new school, you don't know tire ground rules of how you are supposed to behave./