From Longman Dictionary of Contemporary Englishunwrittenun‧writ‧ten /ʌnˈrɪtn/ adjectiveRULE/REGULATIONan unwritten rule, law, agreement etc is one that everyone knows about although it is not official
unwritten rules of social behaviourExamples from the Corpusunwritten• Both sides claimed the other had broken a 1993 unwritten agreement to avoidtargetingcivilians.• Whatever the outcome the long-standing, unwrittencode of behaviour that governsrelations between ministers and civilservants would be gravely battered.• One of the unwritten job descriptions of a basketballcoach 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.
