Source : Webster's Revised Unabridged Dictionary (1913)

Reminder \Re*mind"er\ (-?r), n.
   One who, or that which, reminds; that which serves to awaken
   remembrance.

Source : WordNet®

reminder
     n 1: a message that helps you remember something; "he ignored his
          wife's reminders"
     2: an experience that causes you to remember something
     3: someone who gives a warning so that a mistake can be avoided
        [syn: {admonisher}, {monitor}]