A law, provision, or guideline that is generally accepted but not formally established or enforced. It's become something of an unwritten rule that people from different departments only sit together during lunch. It's just an unwritten rule that a new president will make his previous tax returns public—seems kind of suspicious if they don't. The unwritten rule for many years was that successful athletes were given free passes for bad behavior or poor grades, so long as they remained successful in their sport.