theory |ˈθēərē; ˈθi(ə)rē|noun ( pl. -ries)a supposition or a system of ideas intended to explain something, esp. 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 | 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.PHRASESin 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.ORIGIN late 16th cent. (denoting a mental scheme of something to be done): via late Latin from Greek theōria ‘contemplation, speculation,’from theōros ‘spectator.’