I am really enjoying thumbing through the 2009 book About the Bible: Short Answers to Big Questions (rev.; Augsburg, 2009), by T. Fretheim.
He says that merely seeing the prophet as social reformer or as future-predictor misses the heart of the matter. Here is his succinct definition:
a man or woman dispatched by God with a word of judgment or promise for a critical moment in the history of God’s people. (p 99)