Unfortunately, it's not that simple and there are many scientists who use "theory" in the sense of hypothesis or speculation [see Facts and theories of evolution according to Dawkins and Coyne]. That's not what I want to talk about today.
What do scientists really mean when they refer to "The Theory of Evolution"? There is no single theory of evolution that covers all the mechanisms of evolution. There's the Theory of Natural Selection, and Neutral Theory, and the Theory of Random Genetic Drift, and a lot of theoretical population genetics. Sometimes you can lump them all together by referring to the Modern Synthesis or Neo-Darwinism. These terms are much more accurate than simply saying "The Theory of Evolution" as long as we all understand what those theories mean.
Instead of using the phrase "The Theory of Evolution," I think we should be referring to "evolutionary theory," which may come in different flavors. The term "evolutionary theory" encompasses a bunch of different ideas about the mechanisms of evolution and conveys a much more accurate description of the theoretical basis behind evolution. Douglas Futuyma prefers "evolutionary theory" in his textbook Evolution and I think he's right. It allows him to devote individual chapters to "The Theory of Random Genetic Drift" and "The Theory Natural Selection."
Here's how Futuyma explains the concept of theory in his book Evolution 2nd ed. p. 613.
So is evolution a fact or a theory? In light of these definitions, evolution is a scientific fact. That is, descent of all species, with modification, from common ancestors is a hypothesis that in the past 150 years or so has been supported by so much evidence, and so successfully resisted all challenges, that it has become a fact. But this history of evolutionary change is explained by evolutionary theory, the body of statements (about mutation, selection, genetic drift, developmental constraints, and so forth) that together account for the various changes that organisms have undergone. [my emphasis ... LAM]
We now know that Darwin's hypothesis of natural selection on hereditary variation was correct, but we also know that there are more causes of evolution than Darwin realized, and that natural selection and hereditary variation themselves are more complex than he imagined. A body of ideas about the causes of evolution, including mutation, recombination, gene flow, isolation, random genetic drift, the many forms of natural selection, and other factors, constitute our current theory of evolution, or "evolutionary theory." Like all theories in science, it is a work in progress, for we do not yet know the causes of all of evolution, or all the biological phenomena that evolutionary biology will have to explain. Indeed, some details may turn out to be wrong. But the main tenets of the theory, as far as it goes, are so well supported that most biologists confidently accept evolutionary theory as the foundation of the science of life. p. 14 [my emphasis ... LAM]When you're talking about the mechanisms of evolution, please use "evolutionary theory" instead of "the theory of evolution."
I wish the proponents of the Extended Evolutionary Synthesis would agree that the version of evolutionary theory they wish to extend is the one described by Douglas Futuyma. This would make it easier for them to explain what's wrong with that version and why their proposals are an improvement [see Templeton gives $8 million to prove that there's more to evolution than natural selection].