the·o·ry 
/ˈθi:jəri/ noun
plural the·o·ries /ˈθi:jəri/ noun 1 : an idea or set of ideas that is intended to explain facts or events [count] ▪ a widely accepted scientific theory ▪ Einstein's theory of relativity ▪ theories on/about evolution [noncount] ▪ according to atomic/economic theory 2 [count] : an idea that is suggested or presented as possibly true but that is not known or proven to be true
▪ Her method is based on the theory that all children want to learn. ▪ There are a number of different theories about the cause of the disease. ▪ She proposed a theory of her own. ▪ Investigators rejected the theory that the death was accidental. ▪ There is no evidence to support such a theory. 3 [noncount] : the general principles or ideas that relate to a particular subject
▪ He is a specialist in film theory and criticism. ▪ music theory
▪ Her method is based on the theory that all children want to learn. ▪ There are a number of different theories about the cause of the disease. ▪ She proposed a theory of her own. ▪ Investigators rejected the theory that the death was accidental. ▪ There is no evidence to support such a theory.
▪ He is a specialist in film theory and criticism. ▪ music theory
in theory 1 — used to say what should happen or be true if a theory is correct ▪ In theory, the reading skills of the students should improve. 2 — used to say that something seems to be true or possible as an idea but may not actually be true or possible ▪ I agree with you in theory, but realistically I don't think we have the time to do that.
