The Collaborative International Dictionary of English v.0.48:
Reminder \Re*mind"er\ (-?r), n. One who, or that which, reminds; that which serves to awaken remembrance. [1913 Webster]WordNet (r) 3.0 (2006):
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, reminder]