Theory: 1) a hunch (“I have a theory as to why there’s so much corruption in politics….”); 2) a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment. Such fact-supported theories are not “guesses” but reliable accounts of the real world. The theory of biological evolution is more than “just a theory.” It is as factual an explanation of the universe as the atomic theory of matter or the germ theory of disease. Our understanding of gravity is still a work in progress. But the phenomenon of gravity, like evolution, is an accepted fact. (This second definition comes to us straight from the American Association for the Advancement of Science)
The word itself came to English via Late Latin, which itself borrowed it from the Greek theoria which means “contemplation, speculation”. It would be a logical fallacy, however, to claim that, because the word itself comes from the Greek “to speculate”, all a theory is is a wild speculation.