1. In common, everyday language when you have a theory it means you simply have a guess. It doesn't have to be supported by evidence or fact.
2. In the scientific community, an explanation supported by a evidence and is logically sound.
1. "My theory is that Jane stole the cookie from the cookie jar!" - Jack after he found no cookies in the cookie jar.
2. The theory of Evolution logically explains why life is so diverse, and the evidence that supports it includes fossils, DNA, living specimens, and bacterial research.
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.
"Theories can incorporate facts and laws and tested hypotheses."
"True in fact and theory."