Why is evolution called a "theory"?

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!