Ground Rule Definition

noun
Any of a set of rules adapted to playing conditions in a specific ballpark.
Webster's New World
Any basic principle or guideline for carrying out a specific activity.
The ground rules for running an efficient household.
Webster's New World

(idiomatic, usually in the plural) The basic rules or standards; whatever someone must know before proceeding.

Make sure everybody knows the ground rules for the trip before they leave.
Wiktionary