unwritten rule
unwritten law
unwritten agreement
unwritten tradition
A: Are there any rules about how to dress for the meeting?
A：开会时对穿着有什么规定吗？
B: There are no formal rules, but it's an unwritten rule to wear business casual.
B：没有正式的规定，但商务休闲装是一种不成文的惯例。
1) 未记录在案的
2) 不成文的/默契的
3) 潜规则
1. 未写下的；不成文的
adjective
1. Not recorded in writing; understood or implied without being stated formally.
Synonym: unrecorded, unspoken, tacit