What is a theory?
In the creation vs. evolution debate, proponents of Biblical Creationism often assert that evolution is "only a theory, not a fact." This statement is based on the common, vernacular meaning of the word "theory" and is a misunderstanding of what this term means in science.
"Science" comes from the Latin word for knowledge, and science is the study not of isolated facts, but of ideas. Thus it has a rich terminology for describing ideas. A hypothesis (literally, "put under") is the idea underlying an investigation such as an experiment. A law is a phenomenon that has been observed to take place consistently under particular conditions. A model is a set of interrelated ideas, based on laws and other observations, which together form a representation with predictive and explanatory value. A theory is a model that has stood up under repeated testing. Theories explain factual, empirical observations.
If a fact is "something that is definitely true," then no type of idea in science (such as a hypothesis, law, model, or theory) is excluded from also being a fact. An idea such as evolution can be both a theory and a fact.
However, in common usage, the word theory has a dramatically different meaning; it typically refers to a conjecture or speculation. It is a type of idea that is not even good enough to be called a hypothesis. A simple conjecture can never be said to be "true" in the sense that a fact is true, so the sound bite "evolution is only a theory" was coined to suggest that evolution is no more than an unsupported scientific speculation.