What defines a scientific theory?

A scientific theory is defined as a hypothesis that has been rigorously tested and supported by multiple experiments and substantial evidence. This means that a theory is not just an initial idea; it has undergone extensive scrutiny and has withstood various challenges from experiments designed to test its validity. The reliability and predictive power of a theory are bolstered by the accumulation of evidence over time, making it a well-substantiated explanation for a phenomenon.

In contrast, the other options do not meet the criteria of a scientific theory. A universally accepted fact might describe an observation but lacks the rigorous testing and predictive ability that characterizes a theory. An observation without testing does not form the basis of a theory; it lacks the necessary experimental validation. Finally, speculative ideas that lack any evidence are far from the rigor and support required for classification as a scientific theory. Thus, a scientific theory stands as a robust framework built on tested hypotheses, making understandings of the natural world both reliable and credible.