• A person who speaks for God, or by divine inspiration.
  
• A prophet's main role was to speak for God, proclaimin His truth.

See Amos and Jonah.

Biblical prophets worked for spiritual prposes to serve God and the people not themselves. They usually owned little or nothing. They spoke only what God told them to say - no matter how unpopular the message was.

From the dictionary:

Prophesy:

1. to fortell or predict.
   
2. to fortell by divine inspriration.
   
3. to speak as a mediator between God and man
   
   
   

   Deuteronomy 18:21, 22
   

   You may say to yourselves, "how can we know when a message has not been spoken by the Lord?" If what a prophet proclaims in the name of the Lord does not take place or come true, that is a message the Lord has not spoken. That prophet has spoken presumptuously. Do not be afraid of him.