theory

[thee-uh-ree, theer-ee]

noun

plural

theories 

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

theory

/ 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

theory

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

theory

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.)

Word History and Origins

Origin of theory¹

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

Word History and Origins

Origin of theory¹

C16: from Late Latin theōria, from Greek: a sight, from theōrein to gaze upon

Idioms and Phrases

Idioms

1. in theory, ideally; hypothetically.

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

Synonym Study

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.

Example Sentences

Conversation has swirled online since the first batch of episodes were released earlier this month, with fans sharing strong feelings and theories about what happened both during the season and after filming wrapped in 2024.

Read more on Los Angeles Times

These twin mergers not only provide fresh insight into how black holes form and evolve but also confirm, with unmatched precision, the predictions of Albert Einstein's general theory of relativity.

Read more on Science Daily

This milestone paves the way for future research that could deepen our understanding of the universe's evolution -- or even challenge current scientific theories.

Read more on Science Daily

The theory of the soul is itself incoherent.

Read more on The Wall Street Journal

Although this reaction pathway was known in theory, Nakamura explained that "The real problem was that no one thought it feasible."

Read more on Science Daily

Related Words

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