Wikipedia:Theory

The word "Theory" can often lead to confusion, because it has two conflicting meanings: a theory in everyday talk, and a scientific theory. To clarify things even better, let's compare both and compare as well with other scientific-related terms.

Terms
Fact

An observable fact is something that happens, and that we can confirm by observation or with a basic experiment. For example: the sky is blue. By definition, a fact is always true. However, a fact is defined by the conditions in which the observation or experiment took place, and may not be a fact in other circumstances. Such conditions are often implied. For example, when we said that "the sky is blue", we probably meant "the sky is blue... when seen from planet Earth, at sea level, on day hours, without many clouds and with no significant pollution" (among other things).

Hypothesis

A hypothesis is an attempt to explain a fact. It can be an educated guess, based on known things and comparisons with similar facts. However, a good scientific hypothesis must be falsifiable: it should be something that may be tested and proved to be either right or wrong. It may also be in the form of a prediction: if we try to do X, under Y detailed circumstances, then Z should happen.

Note that when we say that something "must be falsifiable", we only mean that it should be conceivable to do so. A hypothesis may not be currently falsifiable because we lack the proper equipment or technological development to check it, or certain natural circumstances beyond human control must be met, but may be done in the future when those things have changed. A hypothesis is not less valid just for not being capable of being tested right away.

Scientific theory

A theory is a well-substantiated and comprehensive explanation of the reasons an observed fact takes place that way and not in another way. They may be used to make predictions, but they are not predictions themselves.

It is quite a big deal for something to be accepted as a scientific theory. It has quite a strong support and explains several observed facts. Although it is possible for a theory to be changed, it is a rare thing to happen. More often than not, new evidence simply provides further info that adds to the existing theories, without any radical amendments. For example, the original theory of evolution by Charles Darwin did not mention DNA, which was unknown at the time. That new knowledge was incorporated, and nowadays we know the role that DNA plays in the evolution of the species.

Law

A law is a principle that helps to explain the natural world, often provided in the form of an equation.

There is a common misunderstanding that a scientific law is higher in some hierarchy than a scientific theory and that a theory may eventually become a law if it gets enough support. That is wrong. Both are different things: laws are patterns, and theories are general explanations. Theories always remain theories, and laws always remain laws.

Belief

Any statement that can not be proved with a scientific method. A belief may actually turn out to be correct (or incorrect, or remain unverified forever), it simply falls outside of the realm of science.

Theory (casual talk)

Besides the scientific meaning, the word "theory" may also have several other meanings in casual talk. Those are just word meanings, not correct or incorrect unto themselves. However, it may be preferable to avoid them when talking about scientific topics, to avoid misunderstandings. Some examples:
 * General explanations in topics other than science (economic theory, political theory)
 * A guess, educated or not, and may not be an actual hypothesis as defined earlier ("My theory is that the main character will die in the series' finale")
 * Something that has only been formulated in the realm of ideas and lacks proper testing in the real world ("In theory, communism works")
 * A comparison between both things ("I have read many books about this, time to put the theory into practice!")