theory

Universal Words noun (plural theories) 1 [C,U] a formal set of ideas that is intended to explain why sth happens or exists: According to the theory of relativity, nothing can travel faster than light. * The debate is centred around two conflicting theories. 2 [U] the principles on which a particular subject is based: the theory and practice of language teaching * literary / political theory 3 [C] ~ (that) an opinion or idea that sb believes is true but that is not proved: I have this theory that most people prefer being at work to being at home. IDIOMS in theory used to say that a particular statement is supposed to be true but may in fact be wrong: In theory, these machines should last for ten years or more. * That sounds fine in theory, but have you really thought it through? * 'Aren't you supposed to be retired?' 'Yes, in theory.'