Longman Dictionary of Contemporary English homepage

Date: 1500-1600
Language: Late Latin
Origin: theoria, from Greek, from theorein 'to look at'

theory

noun
     
Related topics: Hard Science, Politics
theo‧ry S2 W1 plural theories
1 [countable]HP an idea or set of ideas that is intended to explain something about life or the world, especially an idea that has not yet been proved to be true [↪ theoretical]
theory about/on
different theories about how the brain works
theory of
Darwin's theory of evolution
theory that
the theory that light is made up of waves
2 [uncountable]H general principles and ideas about a subject:
Freudian theory has had a great influence on psychology.
political/economic/literary etc theory
I'm taking a course on political theory.
3

in theory

something that is true in theory is supposed to be true, but might not really be true or might not be what will really happen:
In theory, everyone will have to pay the new tax.
4 [countable] an idea or opinion that someone thinks is true but for which they have no proof
theory that
Detectives are working on a theory that he knew his murderer.

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