Definition of "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

"A scientific hypothesis that survives experimental testing becomes a scientific theory."

noun: a belief that can guide behavior

"The architect has a theory that more is less."

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

"True in fact and theory."