The word theory has many distinct meanings in different fields of knowledge, depending on their methodologies and the context of discussion.

In science a theory is a testable model of the manner of interaction of a set of natural phenomena, capable of predicting future occurrences or observations of the same kind, and capable of being tested through experiment or otherwise verified through empirical observation. For the scientist, "theory" is not in any way an antonym of "fact". For example, it is a fact that an apple dropped on earth has been observed to fall towards the center of the planet, and the theories commonly used to describe and explain this behavior are Newton's theory of universal gravitation (see also gravitation), and the general theory of relativity.

In common usage, the word theory is often used to signify a conjecture, an opinion, a speculation, or a hypothesis. In this usage, a theory is not necessarily based on facts; in other words, it is not required to be consistent with true descriptions of reality. True descriptions of reality are more reflectively understood as statements which would be true independently of what people think about them.

According to the United States National Academy of Sciences,

Some scientific explanations are so well established that no new evidence is likely to alter them. The explanation becomes a scientific theory. In everyday language a theory means a hunch or speculation. Not so in science. In science, the word theory refers to a comprehensive explanation of an important feature of nature that is supported by many facts gathered over time. Theories also allow scientists to make predictions about as yet unobserved phenomena,

Etymology

The word derives from Greek theoria (Jerome), Greek "contemplation, speculation", from "spectator", thea "a view" + horan "to see", literally "looking at a show". There is a second possible etymology that traces the word back to to theion "divine things" instead of thea, reflecting the concept of contemplating the divine organisation (Cosmos) of the nature. It is attested in English since 1592.

Science

In scientific usage, a theory does not mean an unsubstantiated guess or hunch, as it can in everyday speech. A theory is a logically self-consistent model or framework for describing the behavior of a related set of natural or social phenomena. It originates from or is supported by rigorous observations in the natural world, or by experimental evidence (see scientific method). In this sense, a theory is a systematic and formalized expression of all previous observations, and is predictive, logical, and testable. In principle, scientific theories are always tentative, and subject to corrections, inclusion in a yet wider theory, or succession. Commonly, many more specific hypotheses may be logically bound together by just one or two theories. As a rule for use of the term, theories tend to deal with much broader sets of universals than do hypotheses, which ordinarily deal with much more specific sets of phenomena or specific applications of a theory.