If there is a precedent for an action or event, it has happened before, and this can be regarded as an argument for doing it again.
双语例句
例： The trial could set an important precedent for dealing with similar cases.
这次审判能为处理类似案件开创一个重要的先例。
例： There are plenty of precedents in Hollywood for letting people out of contracts.
在好莱坞，存在着允许影视人员解除合同的先例。