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

The results, published in Physical Review Letters, show that the strength of gravity decreases with distance almost exactly as predicted by Newton's equations and later incorporated into Einstein's theory of general relativity.

From Science Daily ● Aug. 25, 2026

Mencken wrote, “is the theory that the common people know what they want and deserve to get it good and hard.”

From The Wall Street Journal ● Aug. 24, 2026

Asked if that could mean, theoretically, the Ohtani could build up to five innings by the end of the season, the manager said: “In theory, yes.”

From Los Angeles Times ● Aug. 23, 2026

In theory, the internal medicine or family medicine doctor oversees your preventive care, ongoing symptoms, chronic conditions, and referrals to specialists.

From Slate ● Aug. 22, 2026

However, President Washington made clear in several letters that his wife was extremely disturbed and had a theory about what happened to Ona.

From "In the Shadow of Liberty" by Kenneth C. Davis

Maybe Bianco really does believe his conspiracy theories.

From Los Angeles Times ● Aug. 25, 2026

"One of the theories is that, the way they interact with each other is key for their exotic properties. The lessons we learn here can be applied to much more complex materials."

From Science Daily ● Aug. 20, 2026

Concertgoers flooded social media with their bizarre conspiracy theories after seeing the star perform at MetLife Stadium in New Jersey, claiming that the singer secretly hired a lookalike to take his place during the performance.

From MarketWatch ● Aug. 18, 2026

How can we fight the proliferation of conspiracy theories?

From The Wall Street Journal ● Aug. 15, 2026

Indeed, it used to be thought that Brunelleschi had produced his demonstration images around 1425 simply because scholars wanted to envisage these images as immediately provoking new art and new theories.

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