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".
"A theory, in science, [is] a very large body of information that's withstood a lot of testing. It probably consists of a number of different hypotheses, many different lines of evidence. And it's something that is very difficult to slay with an ugly fact, as Huxley once put it, because it's just a complex body of work that's been worked on through time."