noun (n)

• a message that helps you remember something(noun.communication)

  Example:
  He ignored his wife's reminders.
  

  source: wordnet30
• an experience that causes you to remember something(noun.cognition)

  source: wordnet30
• someone who gives a warning so that a mistake can be avoided(noun.person)

  Synonym:
  admonisher, monitor
  

  source: wordnet30
• One who, or that which, reminds; that which serves to awaken remembrance.(noun)

  source: webster1913