A prediction is a foretelling of something that is not yet known. This is usually of an event that will occur in the future, although can also be the future discovery of something that has already occurred.
In science, a hypothesis or a theory should offer testable predictions. If those predictions are experimentally shown to be wrong, the hypothesis or theory is modified or discarded. Scientific predictions are based on mathematical or computer models. Applications include weather forecasts and predictions of earthquakes and tsunamis. The earliest such predictions date back to antiquity: Babylonian astronomers could predict eclipses.
Some scientific predictions are based on statistical analysis of data. These predictions have an error range which depends on the amount and quality of the data which is available.
Predictions made on the basis of modelling but where the outcome is already known (i.e. the event has already occurred) are also known as postdictions or retrodictions.
The Bible offers several examples of predictions based on revelation from God, including Joseph's interpretation of Pharaoh's dream (Genesis 41:25-32). The Bible's prophetic books include Isaiah, Jeremiah, Lamentations, Ezekiel and Daniel in the Old Testament and the New Testament's Book of Revelation.
Astrology and occultism
Some predictions are made on the basis of spiritism or astrology. The Bible repeatedly and strongly condems such practices. Such predictions also date back to antiquity; the Babylonians attempted to use their astronomical knowledge to predict the futures of nations. Hellenistic astrology, on which modern Western astrology is based, began to predict the future of individuals based on horoscopes detailing the position of celestial bodies at a certain point in time (often the moment of birth).
Other occult means of prediction include handreading, fortune-telling, making predictions based on Tarot cards or summoning spirits.
The most well-known occult prophet was Michel de Nostredame (1503 – 1566), better known as Nostradamus, whose predictions, published in 1555, took the form of notoriously vague and ambiguous couplets.
Scientific predictions can be quite accurate, such as in the prediction of the trajectory of a moving object and the masses of products in a chemical reaction.
Other scientific predictions, such as weather forecasts, are only approximately correct due to insufficient data on which to make the prediction, random error in the data, inadequately sophisticated models or random variations. At other times predictions are impossible, such as for the three body problem, or for problems subject to the butterfly effect.
Other predictions can be completely wrong; for example, the Jehovah's Witnesses have incorrectly predicted Christ's Second Coming on several occasions (in contravention of Matthew 24:36, etc.). The Millerites did the same in their Great Disappointment. Predictions based on astrology or occultism have repeatedly failed scientific scrutiny, with scientific studies of astrological predictions failing to find anything statistically significant.