reminder

reminder
noun
  1. a thing that causes someone to remember something
    提醒物,令人回忆的东西
    the watch tower is a reminder of the days when an enemy might appear at any moment.
    岗楼使人们回忆起敌人随时可能会出现的日子。
    ■a message or communication designed to ensure that someone remembers something
    提示
    ■a letter sent to remind someone of an obligation, especially to pay a bill
    催单,催缴单
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