the·​o·​ry | \ ˈthē-ə-rē How to pronounce theory (audio) , ˈthir-ē \
plural theories

Definition of theory

1 : a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena the wave theory of light
2a : a belief, policy, or procedure proposed or followed as the basis of action her method is based on the theory that all children want to learn
b : an ideal or hypothetical set of facts, principles, or circumstances often used in the phrase in theory in theory, we have always advocated freedom for all
3a : a hypothesis assumed for the sake of argument or investigation
b : an unproved assumption : conjecture
c : a body of theorems presenting a concise systematic view of a subject theory of equations
4 : the general or abstract principles of a body of fact, a science, or an art music theory
5 : abstract thought : speculation
6 : the analysis of a set of facts in their relation to one another

Choose the Right Synonym for theory

hypothesis, theory, law mean a formula derived by inference from scientific data that explains a principle operating in nature. hypothesis implies insufficient evidence to provide more than a tentative explanation. a hypothesis explaining the extinction of the dinosaurs theory implies a greater range of evidence and greater likelihood of truth. the theory of evolution law implies a statement of order and relation in nature that has been found to be invariable under the same conditions. the law of gravitation

The Difference Between Hypothesis and Theory

A hypothesis is an assumption, an idea that is proposed for the sake of argument so that it can be tested to see if it might be true.

In the scientific method, the hypothesis is constructed before any applicable research has been done, apart from a basic background review. You ask a question, read up on what has been studied before, and then form a hypothesis.

A hypothesis is usually tentative; it's an assumption or suggestion made strictly for the objective of being tested.

A theory, in contrast, is a principle that has been formed as an attempt to explain things that have already been substantiated by data. It is used in the names of a number of principles accepted in the scientific community, such as the Big Bang Theory. Because of the rigors of experimentation and control, it is understood to be more likely to be true than a hypothesis is.

In non-scientific use, however, hypothesis and theory are often used interchangeably to mean simply an idea, speculation, or hunch, with theory being the more common choice.

Since this casual use does away with the distinctions upheld by the scientific community, hypothesis and theory are prone to being wrongly interpreted even when they are encountered in scientific contexts—or at least, contexts that allude to scientific study without making the critical distinction that scientists employ when weighing hypotheses and theories.

The most common occurrence is when theory is interpreted—and sometimes even gleefully seized upon—to mean something having less truth value than other scientific principles. (The word law applies to principles so firmly established that they are almost never questioned, such as the law of gravity.)

This mistake is one of projection: since we use theory in general to mean something lightly speculated, then it's implied that scientists must be talking about the same level of uncertainty when they use theory to refer to their well-tested and reasoned principles.

The distinction has come to the forefront particularly on occasions when the content of science curricula in schools has been challenged—notably, when a school board in Georgia put stickers on textbooks stating that evolution was "a theory, not a fact, regarding the origin of living things." As Kenneth R. Miller, a cell biologist at Brown University, has said, a theory "doesn’t mean a hunch or a guess. A theory is a system of explanations that ties together a whole bunch of facts. It not only explains those facts, but predicts what you ought to find from other observations and experiments.”

While theories are never completely infallible, they form the basis of scientific reasoning because, as Miller said "to the best of our ability, we’ve tested them, and they’ve held up."

Two Related, Yet Distinct, Meanings of Theory

There are many shades of meaning to the word theory. Most of these are used without difficulty, and we understand, based on the context in which they are found, what the intended meaning is. For instance, when we speak of music theory we understand it to be in reference to the underlying principles of the composition of music, and not in reference to some speculation about those principles.

However, there are two senses of theory which are sometimes troublesome. These are the senses which are defined as “a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena” and “an unproven assumption; conjecture.” The second of these is occasionally misapplied in cases where the former is meant, as when a particular scientific theory is derided as "just a theory," implying that it is no more than speculation or conjecture. One may certainly disagree with scientists regarding their theories, but it is an inaccurate interpretation of language to regard their use of the word as implying a tentative hypothesis; the scientific use of theory is quite different than the speculative use of the word.

Examples of theory in a Sentence

The immune surveillance theory of cancer holds that in a way we all do have cancer, that a healthy immune system fights off rogue cells as they appear. — Sallie Tisdale, Harper's, June 2007 The family's theory was that the cheating businessmen somehow framed their brother. — Eliza Griswold, Harper's, September 2006 The theory of the teacher with all these immigrant kids was that if you spoke English loudly enough they would eventually understand. — E. L. Doctorow, Loon Lake, (1979) 1980 While strolling around, we kept the run of the moon all the time, and we still kept an eye on her after we got back to the hotel portico. I had a theory that the gravitation of refraction, being subsidiary to atmospheric compensation, the refrangibility of the earth's surface would emphasize this effect in regions where great mountain ranges occur, and possibly so even-handed impact the odic and idyllic forces together, the one upon the other, as to prevent the moon from rising higher than 12,200 feet above sea-level. This daring theory had been received with frantic scorn by some of my fellow-scientists, and with an eager silence by others. — Mark Twain, A Tramp Abroad, 1880 a widely accepted scientific theory Her method is based on the theory that all children want to learn. There are a number of different theories about the cause of the disease. She proposed a theory of her own. Investigators rejected the theory that the death was accidental. There is no evidence to support such a theory. He is a specialist in film theory and criticism. See More
Recent Examples on the Web In theory, Iraq can pump an extra 660,000 bpd, said Yousef Alshammari, CEO and head of oil research at CMarkits in London. Nadeen Ebrahim, CNN, 9 May 2022 Congress could, in theory, enact protections, although the filibuster is a barrier. Amy Davidson Sorkin, The New Yorker, 8 May 2022 In theory, the expensive liquor brands preferred by wealthy consumers should be able to charge more without hurting demand. Carol Ryan, WSJ, 5 May 2022 In theory, the Supreme Court’s word is supposed to be final, not subject to the same kinds of pressures that influence other powerful public officials. Matt Pearcestaff Writer, Los Angeles Times, 4 May 2022 In theory, this blockbuster promotional model has long gone the way of the dinosaur, or, well, Blockbuster Video. Will Dukes, Rolling Stone, 4 May 2022 In theory, this could allow the driver to better see the display when driving alone or let the passenger enjoy entertainment on the go without distracting the driver (assuming this works while the vehicle is in motion). Mark Knapp, PCMAG, 4 May 2022 The justices on the one hand might argue that public opinion about the Court doesn’t much matter in the sense that, at least in theory, the judiciary is not supposed to be responsive to public opinion. Scott Nover, Quartz, 4 May 2022 As more workers returned to their jobs, for example, employers wouldn't in theory have to keep raising wages. Susan Tompor, Detroit Free Press, 4 May 2022 See More

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'theory.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

First Known Use of theory

1592, in the meaning defined at sense 6

History and Etymology for theory

Late Latin theoria, from Greek theōria, from theōrein

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Last Updated

11 May 2022

Cite this Entry

“Theory.” Dictionary, Merriam-Webster, Accessed 17 May. 2022.

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More Definitions for theory


the·​o·​ry | \ ˈthē-ə-rē How to pronounce theory (audio) , ˈthir-ē \
plural theories

Kids Definition of theory

1 : an idea or opinion that is presented as true Nobody knows where he went, but each of us has a theory. Perhaps they were formulating their own theories about how Cedric had died.— J. K. Rowling, Harry Potter and the Goblet of Fire
2 : a general rule offered to explain a scientific phenomenon the theory of gravity
3 : the general rules followed in a science or an art music theory


the·​o·​ry | \ ˈthē-ə-rē How to pronounce theory (audio) , ˈthi(-ə)r-ē How to pronounce theory (audio) \
plural theories

Medical Definition of theory

1 : the general or abstract principles of a body of fact, a science, or an art the theory and practice of medicine
2 : a plausible or scientifically acceptable general principle or body of principles offered to explain natural phenomena a theory of organic evolution — see atomic theory, cell theory, germ theory
3 : a working hypothesis that is considered probable based on experimental evidence or factual or conceptual analysis and is accepted as a basis for experimentation

Other Words from theory

theoretical \ ˌthē-​ə-​ˈret-​i-​kəl, ˌthi(ə)r-​ˈet-​ How to pronounce theory (audio) \ also theoretic \ -​ik How to pronounce theory (audio) \ adjective
theoretically \ -​i-​k(ə-​)lē How to pronounce theory (audio) \ adverb

More from Merriam-Webster on theory

Nglish: Translation of theory for Spanish Speakers

Britannica English: Translation of theory for Arabic Speakers


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