A supposition or a system of ideas intended to explain something, especially one based on general principles independent of the thing to be explained.
(dictionary definition)
I don't know what dictionary you're using but it's not a supposition.
Now that we've brought up evolution we're talking science, so we need to concentrate on the scientific definition of the word
theory and not its bastardised version found in everyday life (where it means more or less the same as a guess, or a hunch). Forget that - that's just one of the many regrettable examples of where a scientific word has escaped into the wild to be misused by the general population.
Science now, remember?
Think of a theory as being a bit like an umbrella; it's an over-arching conceptual framework which ties together and explains lots of disparate and discrete facts. So, for example, before 1859 lots and lots of people knew lots of different things about evolution, in the sense of heredity and so forth, but they had no ways of explaining those facts. People had been breeding animals for thousands of years, and knew that doing such-and-such typically gave such-and-such a result,
but without knowing why. (To be fair, even Darwin didn't, precisely, because there was no true science of genetics for another century or so). There was nothing to tie them all together and to say "
This is what's actually going on." The theory of evolution explains all those different facts by providing a unified conceptual framework that puts them all in the same basket and explaining what does what and how.
Wikipedia defines a scientific theory as:
"... a well-substantiated explanation of some aspect of the natural world that is traditionally acquired through the scientific method and repeatedly tested and confirmed through observation and experimentation. As with most (if not all) forms of scientific knowledge, scientific theories are inductive in nature and aim for predictive power and explanatory capability ... The strength of a scientific theory is related to the diversity of phenomena it can explain ... Scientific theories are usually testable and make falsifiable predictions. They describe the causal elements responsible for a particular natural phenomenon, and are used to explain and predict aspects of the physical universe or specific areas of inquiry (e.g., electricity, chemistry, astronomy). Scientists use theories as a foundation to gain further scientific knowledge, as well as to accomplish goals such as inventing technology or curing disease. Scientific theories are the most reliable, rigorous, and comprehensive form of scientific knowledge. This is significantly different from the common usage of the word 'theory', which implies that something is a conjecture, hypothesis, or guess (i.e., unsubstantiated and speculative)."
