Okay. I had a disagreement with J the other day about whether or not evolution is a fact. He supports evolution, but claimed that the fact that something is happening doesn't make it a fact. His example was, "if you call someone and tell them I'm at the pizza place, then it's not necessarily a fact to them." (But that doesn't matter because regardless of whether or not that person believed he was at the pizza place, he'd still be at the pizza place)
Another person I talked to agreed with him, stating that the definition of fact mentioned the word truth and she argued that evolution cannot be proven to be an ultimate truth. (But what can? Can't we even argue our own non-existence under this assumption?)
According to Wikipedia, a fact is "something that is the case, something that actually exists, or something that can be verified according to an established standard of evaluation." It goes on to say, "Just as in philosophy, the scientific concept of fact is central to fundamental questions regarding the nature, methods, scope and validity of scientific reasoning. In the most basic sense, a scientific fact is an objective and verifiable observation."
Evolution is a fact in both cases; generally and scientifically. It is the case. It exists. It can be verified. We have mountains of evidence to support it.
The logic isn't too different from that of gravity. Wikipedia says, "...it is a fact that an apple dropped on earth has been observed to fall towards the center of the planet, and the theories commonly used to describe and explain this behaviour are Newton's theory of universal gravitation (see also gravitation), and general relativity."
Likewise, it is a fact that a process (evolution) occurs that results in heritable changes in a population spread over many generations. The theory commonly used to describe and explain this is the theory of evolution, which includes Darwinian theory.
I call evolution a fact because it is...but also because calling it a theory leads to statements like "it's just a theory."