Theory

In common parlance, a theory is a hunch or guess about something (e.g., "I have a theory that my teacher is an alien"). In science, however, a theory is an explanation of a set of observations that has been tested and found to be well-supported by evidence (e.g., "the theory of relativity"). The common usage of the word theory is closer in meaning to hypothesis in science: a plausible (or possible) explanation.

The distinction between a theory and a hypothesis (or even a guess) is an important one, and ignoring it leads to the kind of equivocation in apologetics exemplified by the claim that "evolution is only a theory".

As defined by Kevin Padian in his testimony in the Kitzmiller v. Dover Area School District trial: