A theory is a well-substantiated, comprehensive explanation of some aspect of the natural world or a phenomenon, based on a body of facts that have been repeatedly confirmed through observation, experimentation, and research

. It is not merely a guess or hypothesis but a systematic and rational framework that explains and interprets facts, often incorporating laws, hypotheses, and empirical evidence

. In science, a theory must be testable and falsifiable, meaning it can make predictions that can be confirmed or refuted by further observations or experiments

. Scientific theories are the most reliable and rigorous form of scientific knowledge, providing explanations that are consistent across a broad range of phenomena and supported by multiple independent lines of evidence

. More broadly, a theory can be understood as a set of conceptual tools that enable accurate description, systematic explanation, and knowledge claims about aspects of the world, which can be tested either abstractly or empirically

. Outside of science, the term may also refer to organized principles or beliefs that explain or guide understanding, though these may not always be empirically tested

. In summary, a theory is a carefully developed explanation grounded in evidence that helps us understand, predict, and explain phenomena in a coherent and testable way