Evolution is a theory, not a fact. It's the classic evolution denier's preface to an argument against evolution's validity. Whether it's a case of equivocation bred from disingenuity or ignorance, the imaginary straw man is commonly summoned with the belief that it will stump "secularists;" little do they know it only highlights their lack of a basic understanding of science, or of reality for that matter.
Evolution and theory of evolution can be synonymous but also distinct; evolution can refer to the phenomenon or fact of evolution, and it can also be used as a short way of referring to the theory of evolution. In science, theories are explanations of observable phenomena supported by very extensive testing and evidence, they're not just (un)educated guesses like many assume because of the colloquial use of the word, theory. Many who aren't familiar with the basics of science often find themselves arguing against evolution, not because they necessarily disagree with it but because they simply don't know what it is they're talking about.
When some say, evolution is a theory, not a fact, they're not necessarily, but they usually don't understand why. They assume that because something is a scientific theory, that the subject of said theory hasn't been proved to exist, but this isn't the case. Theories in science explain tested, observable phenomena, things that have happened, matters of fact. The theory of evolution functions with the presupposition that evolution is a reality - it has occurred and does occur all around us. It is the theory of evolution, not the theory on whether or not evolution actually happens. Evolution does happen, that is a fact. And yes, it has been observed - it's observed and even manipulated on a daily basis.
Really, evolution is not that radical an idea. People who breed animals for meat, strength, fur, etc., possess a practical understanding of the evolutionary process, of natural selection - or in this case, it can be called artificial selection, although it's the same thing in principle. This is also the case when people understand that two adults with similar traits will beget children likely to also possess said traits. When a tall couple is spotted and one thinks, "they're going to have tall kids," that's a reference to evolution. When dog breeders choose mates to preserve the species or produce puppies with any desired attributes, that's a reference to evolution. It's really that simple.
Most who deny evolution actually believe in it without even knowing they do. Some who come to terms with this will attempt to distinguish micro and macro evolution, as if this somehow spares them from conceding that evolution is a reality; well, micro and macro evolution are results of the same process, they only differ in how long the process has gone on relative to a particular point in time.
Evolution is a reality, it is fact, and it is a popular name for the theory on the subject. It's not radical by any means. There's no reason for this to be a subject of controversy. Let us accept the reality of it and move on. If people really want to waste their time trying to debunk the subjects of scientific theory, try gravity on for size - that would make for an entertaining debate.