A scientific theory is an explanation of an aspect of the natural world that can be repeatedly tested, in accordance with the scientific method, using a predefined protocol of observation and experiment. Established scientific theories have withstood rigorous scrutiny and embody scientific knowledge.
The definition of a scientific theory (often contracted to "theory" for the sake of brevity) as used in the disciplines of science is significantly different from the common vernacular usage of the word "theory". In everyday speech, "theory" can imply that something is an unsubstantiated and speculative guess, the opposite of its meaning in science. These different usages are comparable to the opposing usages of "prediction" in science versus everyday speech, where it denotes a mere hope.
The strength of a scientific theory is related to the diversity of phenomena it can explain and its simplicity. As additional scientific evidence is gathered, a scientific theory may be modified and ultimately rejected if it cannot be made to fit the new findings; in such circumstances, a more accurate theory is then required. In certain cases, the less-accurate unmodified scientific theory can still be treated as a theory if it is useful (due to its sheer simplicity) as an approximation under specific conditions. A case in point is Newton's laws of motion, which can serve as an approximation to special relativity at velocities that are small relative to the speed of light.
Scientific theories are testable and make falsifiable predictions.[5] They describe the causes of a particular natural phenomenon and are used to explain and predict aspects of the physical universe or specific areas of inquiry (for example, electricity, chemistry, and astronomy). Scientists use theories to further scientific knowledge, as well as to facilitate advances in technology or medicine.
As with other forms of scientific knowledge, scientific theories are both deductive and inductive, aiming for predictive and explanatory power.
The paleontologist Stephen Jay Gould wrote that "...facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world′s data. Theories are structures of ideas that explain and interpret facts.
