约定俗成（約定俗成）　

◎ 约定俗成 yuēdìng-súchéng

[established by usage;accepted through common practice;convention] 事物的名称或法则,经人相约遵用,久而久之,为社会所公认或实用

名无固宜,约之以命,约定俗成谓之宜,异于约则谓之不宜。——《荀子·正名》