This is a pretty good explanation of a scientific theory for our creationist friends who say "but it's only a theory"
Quote:
The word theory in the theory of evolution does not imply doubt from mainstream science regarding its validity; the concepts of theory and hypothesis have specific meanings in a scientific context. While theory in colloquial usage may denote a hunch or conjecture, a scientific theory is a set of principles that explains observable phenomena in natural terms.Evolution is a theory in the same sense as germ theory, gravitation, or plate tectonics