[ thee-uh-ree, theer-ee ]
/ ˈθi ə ri, ˈθɪər i /
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See synonyms for: theory / theories on Thesaurus.com

noun, plural the·o·ries.
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Idioms about theory

    in theory, ideally; hypothetically: In theory, mapping the human genome may lead to thousands of cures.

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 -y3

synonym study for theory

1, 2. 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.


hypothesis, law, theory (see synonym study at the current entry)
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2021

How to use theory in a sentence

British Dictionary definitions for theory

/ (ˈθɪərɪ) /

noun plural -ries
a system of rules, procedures, and assumptions used to produce a result
abstract knowledge or reasoning
a speculative or conjectural view or ideaI have a theory about that
an ideal or hypothetical situation (esp in the phrase in theory)
a set of hypotheses related by logical or mathematical arguments to explain and predict a wide variety of connected phenomena in general termsthe theory of relativity
a nontechnical name for hypothesis (def. 1)

Word Origin for theory

C16: from Late Latin theōria, from Greek: a sight, from theōrein to gaze upon
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Medical definitions for theory

[ thēə-rē, thîrē ]

A systematically organized body of knowledge applicable in a relatively wide variety of circumstances, especially a system of assumptions, accepted principles, and rules of procedure devised to analyze, predict, or otherwise explain the nature or behavior of a specified set of phenomena.
Abstract reasoning; speculation.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

Scientific definitions for theory

[ thēə-rē, thîrē ]

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. See Note at hypothesis.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

Cultural definitions for theory


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.)

The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.