"Prophet" also means "someone who foretells the future." Think of "prophecy," whose primary meaning is a prediction of future events, usually divinely inspired, e.g. the alleged prophecies of Nostradamus. The wormhole aliens exist outside of time, perceiving past, present, and future as one, and thus they can reveal the future (or the past) to the Bajorans through Orb experiences, i.e. prophecies.