If evolution were really true, then it wouldn't be hard at all to "define" it. But since it's merely a theory that exists in the imaginations of men, then its definition will be as numerous and varied as each individual imagination.
If there were evidence, then it would no longer be a theory, but a fact. But since the "evidence" consists men looking at fossils, skulls and bones and imagining what they could be and how they got that way, then again, it's just a theory as you said, not a fact.