The word or phrase Theory refers to a tentative insight into the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena, or a belief that can guide behavior, or 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, or a religious doctrine that is proclaimed as true without proof, or a saying that is widely accepted on its own merits.  Theory definition, translation and meaning of Theory in English. Learn and practice the pronunciation of Theory. Find the answer of what is the meaning of Theory in English.