unwritten
adjective uk/ʌnˈrɪt.ən/ us/ʌnˈrɪt̬.ən/
非书面的;口头的;不成文的
Something that is unwritten does not exist in a written or printed form.
an unwritten constitution 不成文宪法
不成文的，约定俗成的
An unwritten rule or law does not exist officially but people generally accept and obey it.
There's an unwritten rule that you don't wear jeans to work. 上班不穿牛仔裤是条不成文的规定。