There’s an important difference between a scientific theory and the fanciful theories of an imaginative raconteur, and this quirk of semantics can lead to an all-too-common misconception. In general conversation, a ‘theory’ might simply mean a guess. But a scientific theory respects a somewhat stricter set of requirements. When scientists discuss theories, they are designed as comprehensive explanations for things we observe in nature. They’re founded on strong evidence and provide ways to make real-world predictions that can be tested.
While scientific theories aren’t necessarily all accurate or true, they shouldn’t be belittled by their name alone. The theory of natural selection, quantum theory, the theory of general relativity and the germ theory of disease aren’t ‘just theories’. They’re structured explanations of the world around us, and the very foundation of science itself.
There's an extended blog post on the project here: http://www.rigb.org/blog/2014/november/its-just-a-theory