Throughout history, the term Prophet has been used to describe a number of religious figures who have acted as intermediaries for the divine. Often, it refers to individuals who promote change in a society, and those serving as spokesmen of God.
Within the Brotherhood of Nod, the Prophet referred directly to Kane. The title is appropriate, considering Nod's pseudo-religious ideology, and its naming conventions. Kane has built up this image, with a number of myths built around him.
That said, various members of the Brotherhood have declared Kane a false prophet throughout the organization's turbulent history - such dissenters have a tendency to meet an unfortunate end.