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

Espenson’s theory still holds: Chosen One stories still rule.

From Salon ● Aug. 16, 2026

Thomas Malthus promulgated the theory that population would grow at a faster rate than the means to support that population could, and so there would have to be ways to reduce population in unpleasant ways.

From The Wall Street Journal ● Aug. 16, 2026

This is where police and organisers use motorbikes and cars to ensure the road is clear at certain junctions and temporarily enforce a closure in theory until all competitors have passed through.

From BBC ● Aug. 15, 2026

He dismisses the theory, also put forward by the KWS, that the cyanide might have come from a type of foliage, noting local livestock would have died as well.

From Barron's ● Aug. 13, 2026

Unfortunately, the Whole Brainers have no hard evidence for their theory.

From "Phineas Gage" by John Fleischman

How can we fight the proliferation of conspiracy theories?

From The Wall Street Journal ● Aug. 15, 2026

Many physicists see this breakdown as evidence that existing theories do not provide a complete description of the Universe at its earliest stage.

From Science Daily ● Aug. 11, 2026

For me, both theories hold some grain of truth.

From BBC ● Aug. 6, 2026

The statement on Thursday revived those theories, as many wondered why there were no photos of McConnell leaving the rehabilitation center.

From Salon ● Aug. 6, 2026

Lots of times, that ability to compute and calculate information rapidly was helpful, like when he was reading about complex mathematical theories.

From "Beasts of Prey" by Ayana Gray

Related Words

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