theory

American  

[thee-uh-ree, theer-ee] / ˈθi ə ri, ˈθɪər i /

noun

theories plural

1. a coherent group of tested general propositions, commonly regarded as correct, that can be used as principles of explanation and prediction for a class of phenomena.

   Einstein's theory of relativity.

   Synonyms:

   doctrine, law, principle

2. a proposed explanation whose status is still conjectural and subject to experimentation, in contrast to well-established propositions that are regarded as reporting matters of actual fact.

   Synonyms:

   thesis, postulate, concept, notion, idea

3. Mathematics. a body of principles, theorems, or the like, belonging to one subject.

   number theory.

4. the branch of a science or art that deals with its principles or methods, as distinguished from its practice.

   music theory.

5. a particular conception or view of something to be done or of the method of doing it; a system of rules or principles.

   conflicting theories of how children best learn to read.

6. contemplation or speculation.

   the theory that there is life on other planets.

   Synonyms:

   view, deduction, conclusion, judgment, opinion, thought

7. guess or conjecture.

   My theory is that he never stops to think words have consequences.

   Synonyms:

   presumption, supposition, surmise, hypothesis

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idioms

1. in theory, ideally; hypothetically.

   In theory, mapping the human genome may lead to thousands of cures.

theory British  

/ ˈθɪərɪ /

noun

1. a system of rules, procedures, and assumptions used to produce a result

2. abstract knowledge or reasoning

3. a speculative or conjectural view or idea

   I have a theory about that

4. an ideal or hypothetical situation (esp in the phrase in theory )

5. a set of hypotheses related by logical or mathematical arguments to explain and predict a wide variety of connected phenomena in general terms

   the theory of relativity

6. a nontechnical name for hypothesis

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theory Scientific  

/ thē′ə-rē,thîr′ē /

1. A set of statements or principles devised to explain a group of facts or phenomena. Most theories that are accepted by scientists have been repeatedly tested by experiments and can be used to make predictions about natural phenomena.

2. See Note at hypothesis

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theory Cultural  

1. In science, an explanation or model that covers a substantial group of occurrences in nature and has been confirmed by a substantial number of experiments and observations. A theory is more general and better verified than a hypothesis. (See Big Bang theory, evolution, and relativity.)

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Synonym Usage

In technical or scientific use, Theory, principle, and law represent established, evidence-based explanations accounting for currently known facts or phenomena or for historically verified experience: the theory of relativity, the germ theory of disease, the law of supply and demand, the principle of conservation of energy. Often the word law is used in reference to scientific facts that can be reduced to a mathematical formula: Newton's laws of motion. In these contexts the terms theory and law often appear in well-established, fixed phrases and are not interchangeable. In both technical and nontechnical contexts, theory can also be synonymous with hypothesis, a conjecture put forth as a possible explanation of phenomena or relations, serving as a basis for thoughtful discussion and subsequent collection of data or engagement in scientific experimentation in order to rule out alternative explanations and reach the truth. In these contexts of early speculation, the words theory and hypothesis are often substitutable for one another: Remember, this idea is only a theory/hypothesis; Pasteur's experiments helped prove the theory/hypothesis that germs cause disease. Obviously, certain theories that start out as hypothetical eventually receive enough supportive data and scientific findings to become established, verified explanations. Although they retain the term theory in their names, they have evolved from mere conjecture to scientifically accepted fact.

Other Word Forms

Noun Inflected Forms

• theories
  plural

Etymology

Origin of theory

First recorded in 1590–1600; from Late Latin theōria, from Greek theōría “a viewing, contemplating,” equivalent to theōr(eîn) “to view” + -ia noun suffix; see -y ³

Explanation

When you have a theory, you have a set of beliefs or principles that might not be proven yet. Does anyone have a good theory for where missing socks go when you do laundry? A theory is a set of accepted beliefs or organized principles that explain and guide analysis and one of the ways that theory is defined is that it is different from practice, when certain principles are tested. For example, you could be a musician who plays well but who doesn't have a lot of experience with the theory of music. This word is a noun and comes from the Greek theoria, which means "contemplation or speculation."

Example Sentences

See Examples For:

theory theories

Asking whether SARS-CoV-2 emerged through either zoonotic spillover, which is when a virus jumps from animals to people, or through a laboratory-related incident is not inherently a conspiracy theory.

From Salon ● Aug. 2, 2026

He even shifted his focus away from number theory and complex algebraic geometry.

From The Wall Street Journal ● Aug. 1, 2026

Trade Representative could, in theory, consider restrictions on Chinese SOEs from buying sulfur.

From MarketWatch ● Aug. 1, 2026

That in itself will be costly, but in theory the overhang and what is underneath it will be in United's footprint - and the club will be responsible for it.

From BBC ● Aug. 1, 2026

They don’t start with the world; they start with the theory.

From "The Mona Lisa Vanishes" by Nicholas Day

I’m an associate professor of sociology and criminology, and my scholarship focuses on far-right networks and digital radicalization – the exact ecosystem where these conspiracy theories have taken root and spread.

From Salon ● Aug. 2, 2026

They formed different theories about what had happened to the missing 23-year-old, and each theory naturally formed an episode.

From BBC ● Jul. 31, 2026

"I have no doubt whatsoever that the people who espouse theories like those promoted by this individual will continue to disseminate them," Nunez added, referring to some French politicians.

From Barron's ● Jul. 30, 2026

Existing theories of satellite formation offer several possible answers, but recent research into stellar magnetic fields suggests that some of those ideas may need to be reconsidered.

From Science Daily ● Jul. 27, 2026

Newton kicked up a fuss about the use of the word ‘hypothesis’; but there was no comparable debate about theories, facts, or laws of nature.

From "The Invention of Science" by David Wootton

Related Words

• approach
• argument
• assumption
• code
• concept
• doctrine
• idea
• ideology
• method
• philosophy
• plan
• position
• premise
• proposal
• provision
• rationale
• scheme
• speculation
• suspicion
• system