1. 提示;使人回忆起某事的事物
    Something that serves as a reminder of another thing makes you think about the other thing.

    e.g. The British are about to be given a sharp reminder of what fighting abroad really means...
           有些事将很快让英国人清楚意识到海外作战究竟意味着什么。
    e.g. Violence has broken out in the capital, a stark reminder that the religious tensions are refusing to go away.
           首都爆发了暴力事件，这给了人们一个严酷的提醒——宗教紧张局势还在持续。

2. 缴费单;催缴单;催缴通知
    A reminder is a letter or note that is sent to tell you that you have not done something such as pay a bill or return library books.

    e.g. ...the final reminder for the gas bill.
           最后一次煤气催缴通知单