I began my last post with this sentence:
Sadly, there are those who deny the fact of evolution....
Before I even leave the starting gate, many creationist readers will have already snorted in disgust and stopped reading, merely because of my use of the word "fact", instead of "theory". Why did I not use the word "theory"? To be perfectly honest, I used "fact" because Richard Dawkins does, and I admire his forthrightness. But I had a deeper reason, too - and I think it is the same reason that Dawkins chooses to use "fact" as well (and it's not because we happen to believe in its veracity).
It is because the word "theory" causes unnecessary confusion.
What is a theory?
Many would here quote definitions from major dictionaries, trying to make a point about how "theory" has been watered down with a second connotation. I'm not going to, because others have already done the job admirably. Anyone who doesn't know the distinction between the two meanings has only to spend one or two minutes in the culture of evolution and its deniers to learn it.
Instead, I'm going to point something out that should be obvious, but apparently isn't to some. A theory is manmade. It is similar to a book, a song, or even a religion. It is a group of cohesive ideas, knowledge, and descriptions about a subject. But it is made of language, and doesn't actually exist in the strong sense.
However, theories are usually about things that exist (but not always). The things a theory is about, and the theory itself, are discrete. One should not be confused with the other. This is not an overly subtle point - it's about as obvious as saying that we should not confuse the dark side of the moon*, and the Pink Floyd album, Dark Side of the Moon.
* which, like the night side of the Earth, is not always the same side, unless you take "dark" to mean "unseen until mankind launched probes".
Is evolution a theory?
Well, yes, if you are being colloquial or elliptical. The word "evolution" is popularly used to refer to what should more properly be called the "modern evolutionary synthesis". It is also used to refer to Darwin's theory of natural selection. It is shorthand, though, and whenever one uses a shorthand term, things can get confusing. For instance, if you took "evolution" to mean Lamarck's theory of inheritance of acquired characters, "evolution" would be false. So, I wouldn't strictly call "evolution" a theory, anymore than I would call "light" a theory. There are theories about evolution, and about light, but evolution is a process and light is... well, ask a physicist.
By a quirk of language and history, the theory of evolution has come to be called simply "evolution". This has not happened in other cases, such as with light or gravity. But evolution is a process, which either happens or doesn't, in the real world.
You would never hear someone say, "Evolution isn't a fact! It's a word!" But the "only a theory" argument amounts to about as much. This is why I didn't say "theory". Theories are incomplete, and some, like Lamarck's, are demonstrably false. Even Intelligent Design is a theory of evolution, and I deny it.
Evolution is a fact of history. Those who take advantage of the "theory" canard have either not learnt enough about the subject or are being willfully obscurantist. I like to think the former, because then the problem is remediable.