Here is the big secret creationists don't want you to know: the word "theory" has a different definition when used in science than when used in casual conversation! So why is evolution a "theory"? Well, it's not... Not really. It is a fact, in terms of what is plainly observable. It's pretty easy to understand the terminology, which means creationists who try to make people confused about this word are dishonest!
A colleague gave me a great summation recently:
In science, a theory is an explanation for something that is seen in nature. By that definition, evolution is, indeed, a theory because it explains the mechanisms (the how, the why) that lead to the changes that we see in animals (us included).
We recognize, however, that any theory could eventually be replaced with a better theory if that better theory did a better job explaining why we see what we see.
A law is typically used for simply describing some relationship that we see in nature. For example, there’s Charles’s Law that states that for a gas, as the temperature rises, the volume of the gas rises, too. That’s irrefutable. It’s a relationship that has been seen over and over again and holds true all the time.
The theory would be what explains why that law exists. For example, you could have a theory saying that gas particles move around…when you heat them, they move faster and push the walls of the container (like a balloon) outward to cause the volume to increase. The theory explains why or how. The law just tells us the relationship.
The confusion comes when the word theory is used in a non-science way to suggest some guess – sometimes unsupported, often wrong – to explain why something happens. There’s a theory about the 9-11 attacks about Jewish people controlling the world, for example.
In science, a theory has been tested over and over and seems to hold true. Otherwise, it would already have been abandoned.
To me, evolution is not a law because evolution describes the mechanisms of how and why organisms change over time. In all honesty, no scientists disbelieve evolution at this point. There is disagreement on some of the details of the theory – the details of the how and why – but not the big questions that things evolve or even the general ideas of how and why.
This issue was taken up in the July, 2002 issue of Scientific American. Find a web imprint here (but you need to pay for the whole article).
Here is an excerpt on this notion of evolution as a "theory":
1. Evolution is only a theory. It is not a fact or a scientific law.
Many people learned in elementary school that a theory falls in the middle of a hierarchy of certainty--above a mere hypothesis but below a law. Scientists do not use the terms that way, however. According to the National Academy of Sciences (NAS), a scientific theory is "a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses." No amount of validation changes a theory into a law, which is a descriptive generalization about nature. So when scientists talk about the theory of evolution--or the atomic theory or the theory of relativity, for that matter--they are not expressing reservations about its truth.
In addition to the theory of evolution, meaning the idea of descent with modification, one may also speak of the fact of evolution. The NAS defines a fact as "an observation that has been repeatedly confirmed and for all practical purposes is accepted as 'true.'" The fossil record and abundant other evidence testify that organisms have evolved through time. Although no one observed those transformations, the indirect evidence is clear, unambiguous and compelling. All sciences frequently rely on indirect evidence. Physicists cannot see subatomic particles directly, for instance, so they verify their existence by watching for telltale tracks that the particles leave in cloud chambers. The absence of direct observation does not make physicists' conclusions less certain.
I would recommend downloading the whole article for your benefit!