theory

(noun) 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. Examples: "Theories can incorporate facts and laws and tested hypotheses." "True in fact and theory."

(noun) a tentative insight into the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena. Synonyms: hypothesis, possibility. Examples: "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."

(noun) a belief that can guide behavior. Examples: "The architect has a theory that more is less." "They killed him on the theory that dead men tell no tales."