What is a science theory?

In the fields of science a theory is the framework of proven or demonstrative facts. It is never "just a hunch" or hypotheses, which are unproven.
A theory is an explanation. It summarizes a hypothesis based on experiments and tests that have been performed.
Theory" is a Jekyll-and-Hyde term that means different things depending on the context and who is using it. While in everyday speech anything that attempts to provide an explanation for a cause can be dubbed a "theory", a scientific theory has a much more specific meaning.