Well, it’s actually both, though most people don’t realize that the term theory as used in science is nothing like how it is used colloquially. Indeed, it’s rather common for folks that don’t believe in evolution to predicate their doubt on the fact that evolution is “only a theory.” Even a fact is defined differently by scientists from how it is by a layman: within science, a fact is a verified empirical observation (from which evolution has been determined), whereas in vernacular contexts, a fact is assumed to be anything for which there is overwhelming evidence, a standard that would rule out quite a bit of accepted scientific truth. It is largely – though not exclusively – due to this lack of scientific literacy that nearly 44% of Americans disbelieve in evolution, despite the considerable amount of evidence in it’s favor.
Unfortunately, despite my vast interest in this topic, I don’t have much time to get into it as further as I’d like. However, I did find a great page that concisely discusses the nuances and misconceptions about evolution and scientific language. The link is available here, and I definitely encourage you all to take a look (it’s an all-around good blog for matters related to evolution and biology). By far my favorite excerpt from this post is the following:
The honest scientist, like the philosopher, will tell you that nothing whatever can be or has been proved with fully 100% certainty, not even that you or I exist, since we might be dreaming the whole thing. Thus there is no sharp line between speculation, hypothesis, theory, principle, and fact, but only a difference along a sliding scale, in the degree of probability of the idea. When we say a thing is a fact, then, we only mean that the probability of it being true is high—so high that we are not bothered by doubt about it and are ready to act accordingly. By this use of the term “fact”—the only proper definition—evolution is a fact. For the evidence in favor of it is as voluminous, diverse, and convincing as in the case of any other well established fact of science concerning the existence of things that cannot be directly seen, such as atoms, neutrons, or solar gravitation…
This pretty much sums up the proper attitude to have not only towards evolution but with all matters of truth. Nothing can ever be known with certainty no matter how strongly we feel about it (since feelings and intuitions can just as easily be false or malleable). A true pursuer of wisdom and knowledge will know this, own up to it, but nonetheless still be able to assert what is most likely true, based on what we can thus far determine.
In other words, while it’s often foolish to assume with total confidence that something is undeniably true, it’s just as untenable to take a post-modernist view of the world and remain unable to commit to anything as a fact. It’s all about basing your position on as much empirical, logical, and rational evidence as possible, and taking an approach towards knowledge that is a fine balance between humility and confidence.