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"
"true in fact and theory"
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"
"he proposed a fresh theory of alkalis that later was accepted in chemical practices"
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"