A saying about what has not yet happened. Prophecy is recognised only afterward, which is what separates it from a guess.
Greek prophēteia (the gift or function of a prophet), from prophētēs (one who speaks for another, a foreteller): pro- (before) + phēmi (to speak); a prophecy is, by its own root, a speaking-before rather than a knowing-before.
A saying about what has not yet happened, recognisable as prophecy only once the happening arrives. The root does not promise truth, only that the speaking came before the event — the rest is judged afterward.