Gary Farber (you all remember Gary, right? He had a bit of a dry patch a while back, for good reason, but he's been blogging up a storm lately, so go get reacquainted) sent me a link to this nicely done overview of the meaning of the word "theory" that was just published in the Washington Post:

If you want to know one reason why the debate over teaching evolution remains so contentious, consider the stickers some school boards have wanted to paste in high school biology textbooks. They label evolution a "theory, not a fact," suggesting that an alternative explanation is possible.

It's a clever strategy. Even people sympathetic to evolution often don't know how to respond to the assertion that evolution is "just a theory." And the opposite claim -- that evolution is a fact -- can generate skepticism among those who don't like to be told what to think.

But these stickers use the words "theory" and "fact" in a very misleading way. The biggest problem is that "theory" has two separate meanings. In common usage, "theory" means an idea or a hunch: "I have a theory about why she left him." No one really knows what the reasons were, but we can guess.

That's not what "theory" means within science. When scientists speak of the theory of gravitation, cell theory or evolutionary theory, they are talking about scientific concepts that have been so thoroughly tested that they are very unlikely to change. Theories are the results of decades or centuries of scientific effort. They draw on many interconnected observations and ideas. They are the end products of science, not stages on the way to the truth.

In science, a hunch or conjecture is called a hypothesis, not a theory. When Copernicus proposed in the early 16th century that the Earth revolves around the sun rather than vice versa, his idea was a hypothesis. But four centuries of observation and thinking have convinced us that heliocentrism is a theory, not just an intriguing idea. It is compatible with everything we know about the solar system and explains observations that cannot be explained in other ways.

Ideally, English would have a different word for these comprehensive organizing concepts in science. But for now, "theory" is doing double duty. So calling evolution a theory may seem to denigrate it in everyday terms, but in scientific terms that's high praise.

"Fact," on the other hand, is a word that makes many scientists uncomfortable. It implies that something is true beyond doubt. But the defining characteristic of a scientific statement is that its accuracy can be tested by comparing it to observations in the natural world; in other words, a scientific statement must be falsifiable. Accepting that something is true beyond doubt requires an act of faith. But that is a religious matter -- and science cannot aspire to the certainty of religion.

Many scientists are as loose with these terms as nonscientists are. They, too, speak blithely of theories when they mean hypotheses, despite the grief it causes evolutionary biologists. Partly, they are being careless. And maybe they lean toward the term because they know that theories are much more solidly established than are hypotheses. (Your idea may be a hypothesis, but mine is a theory.)

Evolution is not a hypothesis. It is a rich theory, built on hundreds of years of scientific research, that explains why the biological world is the way it is. It provides a mechanism for the continual change of biological organisms over time, beginning with the simplest replicating molecules. It accounts for the simultaneous diversity and unity of the living things we see around us.

The piece hammers on the topic even more, but it's an important concept, and one we have to take back from the creationists. One other important thing: never, ever refer to Intelligent Design creationism as a theory. It isn't one. I'd argue that it doesn't even meet the standards to warrant calling it a hypothesis.

More advice:

Despite the logical flaws in creationist arguments, scientists would be well served by finding more constructive ways to engage in the debate. When creationism is denied a hearing, those who are ambivalent have fewer opportunities to evaluate the evidence. Religion should not be taught in science classes, but suppressing discussion of creationism can create the mistaken impression that scientists have something to hide. And creationism is far from absent in many high school biology classes -- surveys reveal that many teachers already discuss it.

I agree that creationism must be engaged, but we should not do it on terms dictated by the creationists. When asked to "debate", don't bother. Instead, plan an independent and informative lecture or panel discussion on evolution, by competent biologists or other relevant scientists, to parallel or follow up the usual nonsense they will babble about. Rebut them on appropriate terms, don't dignify them by joining their circus.