What’s in a (scientific) theory?

Steve dobbed me in to a friend of his the other night, telling him I was interested in things like evolution. Asked about whether he believed in creationism, the friend said, 'what people seem to forget is that evolution is just a theory'. I was largely saved from having to argue this by the phone ringing, but it echoes things I have heard christian creationists say on various websites and blogs.

Just a theory, say say, waving their hands dismissively. It must be very nice and convenient for them to be able to dismiss evolution this way.

However, all it shows is that they don't understand science. A scientific theory is not the same as when we say 'I've got this theory on why my socks go missing in the wash.' Colloquially, we say theory when we mean speculation, guess or opinion. Scientists say hypothesis when they mean speculation and theory when they mean a model, explanation or description of something that has been observed.

Hence we have the theory of plate tectonics, atomic theory, and the theory of relativity. We have the theory of gravity. We don't just wave our hands and say, 'but gravity is just a theory', as though next time we drop something, it might not fall to the ground.

No, gravity is a fact. A hard solid fact, with absolutely indisputable evidence to prove it. The theory of gravity explains it, models it, and allows us to understand how things behave in gravity. Evolution is also a fact. A hard solid fact, with absolutely indisputable evidence to prove it. Darwin's theory of evolution by natural selection explains it, models it, and allows us to understand how things evolve over time.

So all that happens when creationists trot out this 'evolution is just a theory' nonsense is prove how utterly ignorant they are about the basic principles of science.