The noun theory has 3 senses (first 3 from tagged texts)
1. (62) theory -- (a well-substantiated explanation of some aspect of the natural world; an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena; "theories can incorporate facts and laws and tested hypotheses"; "true in fact and theory")
2. (28) hypothesis, possibility, theory -- (a tentative theory about the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena; "a scientific hypothesis that survives experimental testing becomes a scientific theory"; "he proposed a fresh theory of alkalis that later was accepted in chemical practices")
3. (5) theory -- (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")