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English definition of “theory”

theory

noun [C or U] uk   /ˈθɪə.ri/ us    /ˈθɪr.i/
B2 a formal statement of the rules on which a subject of study is based or of ideas that are suggested to explain a fact or event or, more generally, an opinion or explanation: economic theory scientific theory Darwin's theory of evolution He has a theory that the hole was caused by a meteorite. in theory B2 If something is possible in theory, it should be possible, but often it does not happen in that way: In theory, the journey ought to take three hours, but in practice it usually takes four because of roadworks.
(Definition of theory from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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