Theory (science)
A prediction is a statement or claim that a particular event will occur in the future in more certain terms than a forecast. The etymology of this word is Latin (from pr- "before" plus dicere "to say"). Niels Bohr stated "Prediction is very difficult, especially if it's about the future."

# Informal prediction from hypothesis
Outside the rigorous context of science, prediction is often confused with informed guess or opinion.
A prediction of this kind might be valid and useful if the predictor is a knowledgeable person in the field and is employing sound reasoning and accurate data. Large corporations invest heavily in this kind of activity to help focus attention on possible events, risks and business opportunities, using futurists. Such work brings together all available past and current data, as a basis on which to develop reasonable expectations about the future.

# Opinion Polls
In politics it is common to attempt to predict the outcome of elections (or assess the popularity of politicians) through the use of opinion polls. Prediction games have been used by many corporations and governments to learn about the most likely outcome of future events with amazing accuracy.

# Supernatural (prophecy)
Predictions have often been made, from antiquity until the present, by resorting to paranormal or supernatural means, such as prophecy or by observing omens. Disciplines including water divining, astrology, numerology, and fortune telling, along with many other forms of divination, have been used for centuries or even millennia to predict or attempt to predict the future. So far none of these means of prediction have been proven under controlled conditions and are heavily criticised by scientists and skeptics.

# Anticipatory science forecasts
In a scientific context, a prediction is a rigorous, (often quantitative), statement forecasting what will happen under specific conditions, typically expressed in the form If A is true, then B will also be true. The scientific method is built on testing assertions which are logical consequences of scientific theories. This is done through repeatable experiments or observational studies.
A scientific theory whose assertions are not in accordance with observations and evidence will probably be rejected. Theories that make no testable predictions remain protosciences until testable predictions become known to the community.
Additionally, if new theories generate many new predictions, they are often highly valued, for they can be quickly and easily confirmed or falsified (see predictive power). In many scientific fields, desirable theories are those which predict a large number of events from relatively few underlying principles.
Quantum physics is an unusual field of science because it enables scientists to make predictions on the basis of probability.
Mathematical models and computer models are frequently used to both describe the behaviour of something, and predict its future behaviour.
In microprocessors, branch prediction permits to avoid pipeline emptying at branch instructions. Engineering is a field that involves predicting failure and avoiding it through component or system redundancy.
Some fields of science are notorious for the difficulty of accurate prediction and forecasting, such as software reliability, natural disasters, pandemics, demography, population dynamics and meteorology.

## Example of scientific hypothesis and prediction
In the 1840s the renowned Hungarian physician Ignaz Semmelweis noticed that women giving birth in the Vienna lying-in hospital were dying in one building, but surviving in another.
Upon considering the cause, he found that the surviving women were attended by midwives and not by student physicians. Thus he proposed the hypothesis that the physicians were a factor in the deaths. This proposition impelled Semmelweis to refine the factor. What was the difference between the midwives and the doctors? After more thought, Semmelweis decided that the cadavers which the student doctors were touching must be part of the factor.
What could the doctors do to avoid the factor? Semmelweis predicted that, if the doctors were to wash their hands, then the cadaver factor would be avoided. Semmelweis therefore instructed the student doctors to wash their hands, and the women who were attended by the doctors survived. Thus his prediction was successful, and his hypothesis was validated. (Semmelweis, 1861. The Etiology, Understanding, and Prophylaxis of Childbed Fever)
Other examples abound in the history of science, ranging from expected predictions which did not occur (such as the Michelson-Morley experiment) to new and radical predictions which shockingly confirmed one theory over another (such as the bending of light around the sun seen in the 1919 eclipse, a prediction of Albert Einstein's theory of General relativity).

# Finance
Mathematical models of stock market behaviour are also unreliable in predicting future behaviour. Consequently, stock investors may anticipate or predict a stock market boom, or fail to anticipate or predict a stock market crash.
Some correlation has been seen between actual stock market movements and prediction data from large groups in surveys and prediction games
An actuary uses actuarial science to assess and predict future business risk, such that the risk(s) can be mitigated.
For example, in insurance an actuary would use a life table to predict life expectancy.

# Vision and prophecy
In literature, vision and prophecy are literary devices used to present a possible timeline of future events.  They can be distinguished by vision referring to what an individual sees happen.  The New Testament book of Revelation (Bible) thus uses vision as a literary device in this regard.  It is also prophecy or prophetic literature when it is related by an individual in a sermon or other public forum.
Charles Dickens' A Christmas Carol also makes use of vision as a literary device.  After Scrooge confronts the visions given to him by the Ghosts of Christmas Past, Present, and Yet to Come, he asks whether the future he has seen can be changed.  In other words, he wants to know whether he change the outcome of the ghosts' prophecies.  This question has also been addressed in many science fiction works, particularly those dealing with time travel.

# Prediction in fiction
Fiction (especially fantasy, forecasting and science fiction) often features instances of prediction achieved by unconventional means.
- In fantasy literature, predictions are often obtained through magic or prophecy, sometimes referring back to old traditions. For example, in J. R. R. Tolkien's The Lord of the Rings, many of the characters possess an awareness of events extending into the future, sometimes as prophecies, sometimes as more-or-less vague 'feelings'. The character Galadriel, in addition, employs a water "mirror" to show images, sometimes of possible future events.
- In some of Philip K. Dick's stories, mutant humans called precogs can foresee the future (ranging from days to years). In the story called The Golden Man, an exceptional mutant can predict the future to an indefinite range (presumably up to his death), and thus becomes completely non-human, an animal that follows the predicted paths automatically.
- In the Foundation series by Isaac Asimov, a mathematician finds out that historical events (up to some detail) can be theoretically modelled using equations, and then spends years trying to put the theory in practice. The new science of psychohistory founded upon his success can simulate history and extrapolate the present into the future.
- In Frank Herbert's sequels to Dune, his characters are dealing with the repercussions of being able to see the possible futures and select amongst them. Herbert sees this as a trap of stagnation, and his characters follow a Golden Path out of the trap.
- In Ursula K. Le Guin's The Left Hand of Darkness, the humanoid inhabitants of planet Gethen have mastered the art of prophecy and routinely produce data on past, present or future events on request. In this story, this was a minor plot device.