1.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
2.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
3.a belief that can guide behavior;
the architect has a theory that more is less