Wiktionary
ground rule
n. 1 (context sports English) A rule regarding play on a specific field, course, or court. 2 (context idiomatic usually in the plural English) The basic rules or standards; whatever someone must know before proceeding.
WordNet
ground rule
n. (baseball) a special rule (as in baseball) dealing with situations that arise due to the nature of the playing grounds
Usage examples of "ground rule".
Very well, add to the roles this special ground rule: anything done to a croquet ball by a cat, good or bad, was part of the natural hazards.
He told me he wanted to see me as often as I deemed it necessary, that he would pay whatever fee I required, and he insisted on only one ground rule.