Definition of theory in English:

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Pronunciation: /ˈθɪəri/

noun (plural theories)

1A supposition or a system of ideas intended to explain something, especially one based on general principles independent of the thing to be explained: Darwin’s theory of evolution
More example sentences
  • Instead, he now requires them to be able to explain the scientific theory of evolution.
  • Religious-mystical theories, biological theories and socio-historical theories explain the existence of the caste system.
  • In general, this theory attempts to explain when and how people adopt new behaviors.
1.1A set of principles on which the practice of an activity is based: a theory of education [mass noun]: music theory
More example sentences
  • It was a residential programme which accepted successful applicants for two years of training in the theory of education and practice of teaching.
  • What follows foregrounds just some of the implications of biomedicine for the theory and practice of public mental health.
  • However, in both the Christian and the Islamic worlds, it was the theory, not the practice of music that held sway as an intellectual pursuit.
1.2An idea used to account for a situation or justify a course of action: my theory would be that the place has been seriously mismanaged
More example sentences
  • And of course my personal theory on this is that women, who do childbirth after all, can handle a lot more pain.
  • And of course there is the theory that the dinosaurs were wiped out by a mega-tsunami caused by an asteroid.
  • That's only a theory, of course, but one which I'm willing to defend.
1.3 Mathematics A collection of propositions to illustrate the principles of a subject.
Example sentences
  • His mathematical work covered Cartesian geometry and the theory of equations.
  • This restriction makes the subject very different from the knot theory traditionally studied by mathematicians.
  • Bolzano's theories of mathematical infinity anticipated Georg Cantor's theory of infinite sets.


in theory

Used in describing what is supposed to happen or be possible, usually with the implication that it does not in fact happen: in theory, things can only get better; in practice, they may well become a lot worse
More example sentences
  • The precautionary principle sounds good in theory, but in practice it is a nightmare.
  • Radar also uses microwaves, so that in theory it would be possible to cook food by putting it at the focus of a radar dish.
  • The software makes it possible, in theory, to see and manage files on any storage system or server.


Late 16th century (denoting a mental scheme of something to be done): via late Latin from Greek theōria 'contemplation, speculation', from theōros 'spectator'.

Words that rhyme with theory

beery, bleary, cheery, dearie, dreary, Dun Laoghaire, eerie, eyrie (US aerie), Kashmiri, leery, peri, praemunire, query, smeary, teary, weary

For editors and proofreaders

Line breaks: the¦ory

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