prophet
A prophet is, in etymological terms, simply one who speaks up.
In religion, a prophet is someone who speaks God's truth. Jeremiah, Ezekiel, Isaiah, are all prophets in the Hebrew scriptures.
In Islam Muhammad is referred to as "the Prophet" (capital "p"). Likewise with Joseph Smith, among Mormons.
The prophetic tradition is that of proclaiming the truth without fear of consequences, and of saying things, generally, that people don't want to hear.