View synonyms for theory


[ thee-uh-ree, theer-ee ]


, plural the·o·ries.
  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


/ ˈθɪərɪ /


  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


/ thēə-rē,thîrē /

  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


  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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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

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Word History and Origins

Origin of theory1

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

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Idioms and Phrases

  1. in theory, ideally; hypothetically:

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

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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.

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Example Sentences

“Our prosecutors have all too often inserted themselves into the political process based on the flimsiest of legal theories,” Barr went on.

From Vox

Turn Wilson’s mathematical crank, and you get a related theory describing groups of those pieces — perhaps billiard ball molecules.

She also learns immediately that this theory is “not just incorrect but hateful, like saying that different races had different IQs” — and yet, “in my heart, I knew that Whorf was right,” that language does change the way you think.

From Vox

It applies a different random error to each piece of information that’s encoded—which in theory makes it impossible to break without knowing the key.

From Fortune

Kindrachuk also works on ebola, and he says over the years many such theories have been put forth in scientific journals without provoking this kind of response.

But at the heart of this “Truther” conspiracy theory is the idea that “someone” wants to destroy Bill Cosby.

Is it sort of evidence of the Gladwellian 10,000 hours theory?

But a 2011 study of genetic evidence from 30 ethnic groups in India disproved this theory.

But, in theory, that started to change last week with the first meeting of SIX, the State Innovation Exchange.

So I was happy to see that the European theory of terroir was in action, promoting with pride the qualities of a specific region.

In the year of misery, of agony and suffering in general he had endured, he had settled upon one theory.

Dean Swift was indeed a misanthrope by theory, however he may have made exception to private life.

The other is the new theory: that the Bible is the work of many men whom God had inspired to speak or write the truth.

The evolution theory alleges that they were evolved, slowly, by natural processes out of previously existing matter.

And our surroundings at that particular moment were not the most favorable to coherent thought or plausible theory-building.


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Definitions and idiom definitions from Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.