My favorite explanation so far of theory is below. I use it when talking to believers now.
taken from:
To truly understand evolution, you must first understand science. Unfortunately, one of the most misused words today is also one of the most important to science: theory. Many incorrectly see theory as the opposite of fact. The National Academy of Sciences provides concise definitions of these critical words: A fact is a scientific explanation that has been tested and confirmed so many times that there is no longer a compelling reason to keep testing it; a theory is a comprehensive explanation of some aspect of nature that is supported by a vast body of evidence generating testable and falsifiable predictions.
In science, something can be both theory and fact. We know the existence of pathogens is a fact; germ theory provides testable explanations concerning the nature of disease. We know the existence of cells is a fact and that cell theory provides testable explanations of how cells function. Similarly, we know evolution is a fact and that evolutionary theories explain biological patterns and mechanisms. The late Stephen Jay Gould said it best: “Evolution is a theory. It is also a fact. And facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world’s data. Theories are structures of ideas that explain and interpret facts.”