ground rule
n.
► Sports. A rule governing the playing of a game on a particular field, course, or court.
► A basic rule of procedure or behavior. Often used in the plural.

[TahlilGaran] American Dictionary ▲

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.

[TahlilGaran] English Idioms Dictionary ▲