What is Prophesy?
This is a huge part of answering this question because there are actually two different categories when speaking of prophecy. There is "Prophecy" and there is "prophecy".
Prophecy
In the Old Testament, there were men called Prophets who spoke the very words of God. They would say things like "Thus saith the Lord..." In fact, they wrote down the words that they spoke and their writings became part of the Bible and we treat them with God's authority to this day. These are men like Moses, Isaiah, Jeremiah, Ezekiel, Daniel, and others. In the New Testament, we see men who did similar things. We call them Apostles. This includes Peter, James, John, Paul, and others.
This kind of Prophecy (upper case P) is fully authoritative, 100% accurate, and always to be obeyed. This kind of Prophecy produces Scripture. This kind of Prophecy can be defined as "Declaring the very Words of God."