From Longman Dictionary of Contemporary Englishunwrittenun‧writ‧ten /ʌnˈrɪtn/ adjective RULE/REGULATIONan unwritten rule, law, agreement etc is one that everyone knows about although it is not official unwritten rules of social behaviour
Examples from the Corpus
unwritten• Both sides claimed the other had broken a 1993 unwritten agreement to avoid targeting civilians.• Whatever the outcome the long-standing, unwritten code of behaviour that governs relations between ministers and civil servants would be gravely battered.• One of the unwritten job descriptions of a basketball coach is as second father to the players.• By an unwritten rule, they avoided controversy for the sake of good fellowship.• Without unwritten rules civilised life would be impossible.• Norms can be thought of as unwritten rules. 