ground rule

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Sports A rule governing the playing of a game on a particular field, course, or court.
• noun A basic rule of procedure or behavior.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun sports A rule regarding play on a specific field, course, or court.
• noun idiomatic, usually plural The basic rules or standards; whatever someone must know before proceeding.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (baseball) a special rule (as in baseball) dealing with situations that arise due to the nature of the playing grounds