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theory

[thee-uh-ree, theer-ee]
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noun, 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.
  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.
  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.
  7. guess or conjecture: My theory is that he never stops to think words have consequences.
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Idioms
  1. in theory, ideally; hypothetically: In theory, mapping the human genome may lead to thousands of cures.
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Origin of theory

1590–1600; < Late Latin theōria < Greek theōría a viewing, contemplating, equivalent to theōr(eîn) to view + -ia -y3
Can be confusedhypothesis law theory (see synonym study at the current entry)

Synonym study

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.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for theories

theory

noun plural -ries
  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 ideaI 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 termsthe theory of relativity
  6. a nontechnical name for hypothesis (def. 1)
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Word Origin

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

Word Origin and History for theories

theory

n.

1590s, "conception, mental scheme," from Late Latin theoria (Jerome), from Greek theoria "contemplation, speculation, a looking at, things looked at," from theorein "to consider, speculate, look at," from theoros "spectator," from thea "a view" + horan "to see" (see warrant (n.)). Sense of "principles or methods of a science or art (rather than its practice)" is first recorded 1610s. That of "an explanation based on observation and reasoning" is from 1630s.

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Online Etymology Dictionary, © 2010 Douglas Harper

theories in Medicine

theory

(thēə-rē, thîrē)
n.
  1. 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.
  2. Abstract reasoning; speculation.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

theories in Science

theory

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

theories in Culture

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.)

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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.