A theory that explains an event or set of circumstances as the result of a secret plot by usually powerful conspirators."

That's a fair description - note that it leaves it wide open as to whether in fact it might actually be true in a particular case. While most theories about conspiracies are rubbish, and frequently pernicious rubbish, I think it’s fair enough to assume that an enormous amount of what happens in public life does involve our rulers concealing significant facts and misleading the public - in effect, involved in a conspiracy.

So many times a few years on, when the fuller version comes out, we learn about that kind of stuff.