A scientific theory is a plausible and consistent explanation for observable phenomena. While a scientific law describes something that always happens in the same way, a scientific theory explains what causes it to do so.
For example, doctors have long known that being near a person who has a communicable disease exposes you to a risk of catching it (see quarantine). It was a "law" of contagion that staying far away protected you from catching the disease, even before Pasteur's germ theory of disease provided the modern scientific explanation (the "theory").
A successful theory is most consistent with the actual behavior of the universe. When a phenomenon is discovered that is inconsistent with the prevailing theory, the theory is either revised or thrown out entirely. An example of this is Newtonian mechanics—while Newton's 3 laws of motion provide extremely accurate descriptions/predictions of the behavior of medium-sized objects at low speeds, they are inconsistent with the behavior of very large, very fast, or very small objects. Newtonian mechanics has been shown to be a limiting case of the theory of relativity (in the limit where velocity goes to 0) and quantum mechanics (in the limit where Planck's constant goes to 0, essentially the limit of large objects). Usually a theory which is inconsistent with some experimental evidence is not completely falsified; it is instead shown to be a simplified version of a more complete theory.
A theory is more substantial than a conjecture or hypothesis, but may not be verified as consistent with empirical data.[1] Controversially, some scientists have declared that causes other than natural, observable ones should not be examined - and that theories involving such causes are "not scientific", e.g., behavioral psychology. This limitation and its implications for the philosophy of science plays a large role in the origins debate.
For many philosophers of science, a theory must be falsifiable to be considered scientific. This means that there must be some way to do experiments that could counter the theory's predictions, thus disproving the current theory.[2]
Generally accepted scientific theories have survived over time, have evolved when appropriate, and modified toward consistency with newly discovered data, have not been shown to be false and can make predictions about natural phenomena.[3] Scientific theories can sometimes be used as the basis for industrial and technical developments.
A scientific theory should have to have strong experimental support or accepted by the scientific community. However, some scientists mistakenly refer to untested theories and competing theories. Theories are generally extremely well-confirmed, such as conservation of energy. Some more speculative 'theories' are more accurately labelled 'hypothesis' within the scientific community, such as String Theory.
Some evolutionists frequently claim that the word "theory" means very different things to scientists and non-scientists. For example, a PBS TV show says:[4]
The American Museum of Natural History exhibit on Darwin says:[5]
In fact, it is hard to find anyone who makes this distinction, outside those who are promoting the theory of evolution to the general public.
An example of the term 'theory' in scientific literature is in the field of speculative physics, which proposes something commonly deemed 'String Theory'. At its most basic level, string theory states that the entirety of the universe is made up of tiny, multi-dimensional strings. However, this proposal cannot currently be verified or refuted by modern science - that is, scientists cannot determine if this is right or wrong. Since String Theory makes no verifiable predictions about the universe, either, scientists cannot say that String Theory fits any facts we currently know. Therefore, String Theory is not the sort of scientific theory which is evidenced by facts and must make correct, verifiable predictions about the future.[6]
Surprisingly, modern methods of scientific inquiry were not generally accepted until the late sixteenth and early seventeenth centuries. Thus in terms of human history, scientific inquiry is in its infancy. What Is Scientific Inquiry? - Arizona Collaborative for Excellence in Preparation of Teachers (ACEPT)