[noun] - a formal statement of the rules on which a subject of study is based or of ideas which are suggested to explain a fact or event or, more generally, an opinion or explanationeconomic theory [U]scientific theory [U]music theory [U]Einstein's theory of relativity [C]Darwin's theory of evolution [C]His theory is that the hole was caused by a meteorite. [C + that clause]In theory (= According to the facts), the journey ought to take three hours, but in practice (= when it happens) it usually takes four because of roadworks.
Meaning of THEORY in English
Cambridge English vocab. Кембриджский английский словарь. 2012