It’s not just that there is a misunderstanding about the meaning of theory.
Here’s a handy definition of scientific theory by T. Ryan Gregory: “a theory in science, again following the definition given by the NAS, is “a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses.” Science not only generates facts but seeks to explain them, and the interlocking and well-supported explanations for those facts are known as theories. Theories allow aspects of the natural world not only to be described, but to be understood. Far from being unsubstantiated speculations, theories are the ultimate goal of science.”
However creationist/intelligent design proponents use the “it’s just a theory” argument as if evolution was nothing more than a vague passing idea in someones head. Perhaps they should be told that aerodynamics and aircraft engineering are “just theories” whenever they travel by air – preferably in the first hostess announcement after takeoff!
Some people react to the “just a theory” dismissal of evolution by claiming it to be a “law” or even a “fact.” I think this is inappropriate for describing the whole body of knowledge described by the word evolution.
Our knowledge about evolution includes facts (e.g., fossil records, genetics, molecular biology of DNA), theories (e.g, natural selection, sexual selection, genetic drift) and speculation (e.g., much of evolutionary psychology). Just like any other body of scientific knowledge. Consider atomic science. This includes facts (e.g, brownian motion, atomic spectra, nuclear and chemical reactions), theories( e.g, quantum theory, standard atomic model) and speculation (e.g., string theories).
This mixture of facts, theories and speculation is quite normal for any living science. And that is what evolution is.
So I consider evolution to be more that theory. It also includes facts, hypotheses, ideas and speculation.
I prefer to refer to it as “evolutionary science.”