Evolution is only a theory
Apologists sometimes claim that evolution is a "theory" not a "fact".
- "Evolution is not a fact. That’s why it’s called a theory! There’s more evidence that the Bible is true. "
"Charles Darwin never thought of evolution as anything other than a theory. He hoped that some day it would be proven by the fossil record but did not live to see that, nor have we [emphasis mine].[...] But no one is pointing out that the textbooks will need to be changed because the old theory of evolution taught for 77 years in the classrooms of America as fact is suddenly replaced by a new theory, or I hasten to add, I am sure we will be told a new fact. [...] And now that we have recognized evolution as a theory, I would simply and humbly ask, can we teach it as such and can we also consider teaching other theories of the origin of species?"
In common parlance, a theory is an unsupported idea or a "hunch" — e.g., "I have a theory that restaurants make more money off of skinny customers." A scientist would call this a hypothesis, or maybe a conjecture.
A scientific theory, on the other hand, is an explanation of some aspect of the real world that is well-supported by evidence. At its core, theory really just means explanation. A hypothesis may become a theory once it has been thoroughly tested through experimentation and has not been disproved, but it will never become a fact, no matter how many tests it passes. The "fact" is the observed aspect of nature itself.
Sometimes it is valid to call something both a fact and a theory. One example of this is gravity. This is the name given to the phenomenon whereby massive bodies are attracted to one other. For example, the moon and the Earth are attracted to each other, which is why they don't fly apart as they move through space. This phenomenon is an observed fact: Henry Cavendish actually measured the force of attraction between two lead spheres back in 1797. Different theories for why and how this phenomenon occurs have been put forth. Newton's theory of gravitation (that it is a force acting instantaneously at a distance) was accepted for centuries until Einstein's general theory of relativity completely changed our understanding of gravity (now considered a warping of space-time). In this sense, gravity is both a fact and a theory.
The same can be said for evolution. If evolution is defined as "allele frequency change in a population over generations" (or, more simply, "populations change over time"), then it is an established fact; not even young-Earth creationists can deny that this takes place. On the other hand, the theory of evolution is a scientific theory that ties together evidence of the types of changes that we see taking place in nature, as well as evidence from fossils, genes, proteins, and so on, to explain why and how evolution happens.
For those with a little more elementary scientific knowledge the argument often takes the form "Evolution is only a theory, not a law." This version relies on a misconception that science places theories and laws in an hierarchy, with the latter on top of a factual chain. In fact, no such hierarchy exists; a theory remains a theory and never becomes a law. Laws are used to describe a fact (i.e. Newton's law of universal gravitation) and theories to explain them (i.e. Einstein's theory of general relativity). It is the difference between what and why.