theory

noun

the·​o·​ry ˈthē-ə-rē How to pronounce theory (audio)
ˈthir-ē
plural theories
1
: a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena
the wave theory of light
2
a
: 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
3
a
: 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

Did you know?

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."

Did you know?

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.

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

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 At these schools, new theories are constantly being explored, helping students to expand on their studies further. Quora, Forbes, 10 Feb. 2024 During closing arguments, defense attorney Robin Bernstein-Lev presented a new theory of what happened that night: that Amie may have attacked Pursehouse. Erin Moriarty, CBS News, 10 Feb. 2024 However, there’s little evidence to support this theory. Lauren Manaker Ms, Rdn, Health, 9 Feb. 2024 For his part, Trump’s lawyer seemed more ready to spar on the legal theory than the facts of Jan. 6. TIME, 8 Feb. 2024 And, dogs don’t just do it as an indicator of joy: A new study published in Biology Letters reviewed the existing body of literature to outline several theories pinpointing the mechanisms behind the infamous tail wag. Max Bennett, Discover Magazine, 7 Feb. 2024 Since the critters are not flying directly toward the light, and the behavior has also been observed in species that are not migratory and do not use compass cues, these old theories no longer seem likely, Fabian said. Taylor Nicioli, CNN, 7 Feb. 2024 The organization pointed to a common theory on its website that fluoride might cause cells in growth plates — areas where osteosarcomas typically develop — to become cancerous because fluoride tends to collect in those areas. Jeff A. Chamer, Charlotte Observer, 7 Feb. 2024 Jimmy Kimmel broke down the QAnon-like meltdown currently racing through the conservative media world on Tuesday night (Jan. 30), pointing out the outrageous tin foil hat-esque theories being amplified on Fox News and other right-wing outlets. Gil Kaufman, Billboard, 31 Jan. 2024 See More

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'theory.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History

Etymology

Late Latin theoria, from Greek theōria, from theōrein

First Known Use

1592, in the meaning defined at sense 6

Time Traveler
The first known use of theory was in 1592

Dictionary Entries Near theory

Cite this Entry

“Theory.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/theory. Accessed 24 Feb. 2024.

Kids Definition

theory

noun
the·​o·​ry ˈthē-ə-rē How to pronounce theory (audio)
ˈthi(-ə)r-ē
plural theories
1
: the general ideas or principles of an art or science
music theory
2
: a general principle or set of principles that explains facts or events of the natural world
wave theory of light
3
: a belief, policy, or procedure proposed or followed as the basis of action
her method is based on the theory that all dogs can be trained
b
: an ideal set of facts, principles, or circumstances
often used in the phrase in theory
in theory, everyone can receive a score of 100 on the test
4
: an idea that is the starting point for argument or investigation
the theory of relativity
Etymology

from Latin theoria "a looking at or considering of facts, theory," from Greek theōria "theory, action of viewing, consideration," from theōrein "to look at, consider," — related to theater

Medical Definition

theory

noun
plural theories
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
theoretical adjective
also theoretic
theoretically adverb

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