Evolution is only a theory

"Evolution is only a theory, not a fact."

This argument relies on equivocation between two meanings of theory: in common parlance, a theory is an unsupported idea or a "hunch" &mdash; 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.

Evolution is both a fact and a theory
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.