From WordNet (r) 1.6 [wn]:

  theory
       n 1: an organized system of accepted knowledge that applies in a
            variety of circumstances to explain a specific set of
            phenomena; "true in fact and theory"
       2: a concept that is not yet verified but that if true would
          explain certain facts or phenomena; "he proposed a fresh
          theory of alkalis that later was accepted in chemical
          practices" [syn: {hypothesis}, {possibility}]
       3: a belief that can guide behavior; "the architect has a
          theory that more is less"; "they killed him on the theory
          that dead men tell no tales"