a 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
a set of principles on which the practice of an activity is based:a theory of education [mass noun]:music theory
an idea used to account for a situation or justify a course of action:my theory would be that the place has been seriously mismanaged
Mathematics a collection of propositions to illustrate the principles of a subject.
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
