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.
例句
The trial could set an important precedent for dealing with large numbers of similar cases...這次審判能為處理大量類似案例開創(chuàng)重要的先例。There are plenty of precedents in Hollywood for letting people out of contracts.在好萊塢，允許影視人員解除合同的先例有很多。