noun the·o·ry \ˈthē-ə-rē, ˈthir-ē\

Definition of theory



  1. 1 :  the analysis of a set of facts in their relation to one another

  2. 2 :  abstract thought :  speculation

  3. 3 :  the general or abstract principles of a body of fact, a science, or an art music theory

  4. 4a :  a belief, policy, or procedure proposed or followed as the basis of action her method is based on the theory that all children want to learnb :  an ideal or hypothetical set of facts, principles, or circumstances —often used in the phrase in theory in theory, we have always advocated freedom for all

  5. 5 :  a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena the wave theory of light

  6. 6a :  a hypothesis assumed for the sake of argument or investigationb :  an unproved assumption :  conjecturec :  a body of theorems presenting a concise systematic view of a subject theory of equations

Examples of theory in a sentence

  1. The immune surveillance theory of cancer holds that in a way we all do have cancer, that a healthy immune system fights off rogue cells as they appear. —Sallie Tisdale, Harper's, June 2007

  2. The family's theory was that the cheating businessmen somehow framed their brother. —Eliza Griswold, Harper's, September 2006

  3. The theory of the teacher with all these immigrant kids was that if you spoke English loudly enough they would eventually understand. —E. L. Doctorow, Loon Lake, (1979) 1980

  4. While strolling around, we kept the run of the moon all the time, and we still kept an eye on her after we got back to the hotel portico. I had a theory that the gravitation of refraction, being subsidiary to atmospheric compensation, the refrangibility of the earth's surface would emphasize this effect in regions where great mountain ranges occur, and possibly so even-handed impact the odic and idyllic forces together, the one upon the other, as to prevent the moon from rising higher than 12,200 feet above sea-level. This daring theory had been received with frantic scorn by some of my fellow-scientists, and with an eager silence by others. —Mark Twain, A Tramp Abroad, 1880

  5. a widely accepted scientific theory

  6. Her method is based on the theory that all children want to learn.

  7. There are a number of different theories about the cause of the disease.

  8. She proposed a theory of her own.

  9. Investigators rejected the theory that the death was accidental.

  10. There is no evidence to support such a theory.

  11. He is a specialist in film theory and criticism.

Recent Examples of theory from the web

These example sentences are collected from online sources. Help us improve them by sending feedback.

Two related, yet distinct, meanings of theory

There are many shades of meaning to the word theory. Most of these are used without difficulty, and we understand, based on the context in which they are found, what the intended meaning is. For instance, when we speak of music theory we understand it to be in reference to the underlying principles of the composition of music, and not in reference to some speculation about those principles.

However, there are two senses of theory which are sometimes troublesome. These are the senses which are defined as “a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena” and “an unproven assumption; conjecture.” The second of these is occasionally misapplied in cases where the former is meant, as when a particular scientific theory is derided as "just a theory," implying that it is no more than speculation or conjecture. One may certainly disagree with scientists regarding their theories, but it is an inaccurate interpretation of language to regard their use of the word as implying a tentative hypothesis; the scientific use of theory is quite different than the speculative use of the word.

Origin and Etymology of theory

Late Latin theoria, from Greek theōria, from theōrein

First Known Use: 1592

Synonym Discussion of theory

hypothesis, theory, law mean a formula derived by inference from scientific data that explains a principle operating in nature. hypothesis implies insufficient evidence to provide more than a tentative explanation a hypothesis explaining the extinction of the dinosaurs. theory implies a greater range of evidence and greater likelihood of truth the theory of evolution. law implies a statement of order and relation in nature that has been found to be invariable under the same conditions the law of gravitation.

THEORY Defined for English Language Learners


noun the·o·ry \ˈthē-ə-rē, ˈthir-ē\

Definition of theory for English Language Learners

  • : an idea or set of ideas that is intended to explain facts or events

  • : an idea that is suggested or presented as possibly true but that is not known or proven to be true

  • : the general principles or ideas that relate to a particular subject

THEORY Defined for Kids


noun the·o·ry \ˈthē-ə-rē, ˈthir-ē\

Definition of theory for Students



  1. 1 :  an idea or opinion that is presented as true Nobody knows where he went, but each of us has a theory. Perhaps they were formulating their own theories about how Cedric had died. — J. K. Rowling, Goblet of Fire

  2. 2 :  a general rule offered to explain a scientific phenomenon the theory of gravity

  3. 3 :  the general rules followed in a science or an art music theory

Medical Dictionary


play play
noun the·o·ry \ˈthē-ə-rē, ˈthi(-ə)r-ē\

Medical Definition of theory



  1. 1:  the general or abstract principles of a body of fact, a science, or an art the theory and practice of medicine

  2. 2:  a plausible or scientifically acceptable general principle or body of principles offered to explain natural phenomena a theory of organic evolution—see atomic theory, cell theory, germ theory

  3. 3:  a working hypothesis that is considered probable based on experimental evidence or factual or conceptual analysis and is accepted as a basis for experimentation


\ˌthē-ə-ˈret-i-kəl, ˌthi(ə)r-ˈet-\play also


\-ik\play adjective


\-i-k(ə-)lē\play adverb

Seen and Heard

What made you want to look up theory? Please tell us where you read or heard it (including the quote, if possible).


capable of being understood in two ways

Get Word of the Day daily email!


Take a 3-minute break and test your skills!

Name That Thing

Test your visual vocabulary with our 10-question challenge!


Test Your Knowledge - and learn some interesting things along the way.