Conspiracy theories just need to be theories of conspiracy. It carries an implication of implausibility only because of popular perception. e.g. TV and movies like to show a "conspiracy theorist" as a person struggling with paranoia who has outlandish theories and no credibility.
Real, mundane conspiracies are numerous, and there are probably some outlandish ones, too. Every proven conspiracy must have started with someone's theory, if not an unprompted admission, so it's not always wrong to theorize. It's just silly to dedicate more than a little brain space*time to theories you've got no evidence for or hope of proving, conspiracy or otherwise.
You are correct about the dictionary definition of the word, but in popular parlance the phrase "conspiracy theory" has a massive negative connotation. I have literally never in my life seen someone use the phrase without intending to imply "crazy/unprovable/etc".
As a sort of recursive aside, the particular strength of the modern connotations of conspiracy theory are supposed to be a result of CIA propaganda produced to help cover up mkUltra.
The common parlance is very problematic. Anyone can see it is in fact a conspiracy theory, but if it's true, you're not supposed to call it that? How would you know?
Using conspiracy theory as a derogaratory term is a form of doublespeak that shield the conspirators and their conspiracies.
I, for one, will not stop calling conspiracy theories conspiracy theories, regardless whether they are true. Actually, especially when they are true.
"It's not a conspiracy if it is not illegal." By definition.
Garden-variety corruption doesn't need a conspiracy. The US is today the world leader in high-level, fully legal corruption. It took many decades to get there, but it will take even longer to choke it off, if we ever do.
We even elected an out-and-out con man to the presidency. Russia wishes it could be so corrupt, but just doesn't have the money for it. China does, but its corruption is mostly still technically illegal.
A lot of garden variety corruption such as this is actually illegal in the US.
However, you can also form a conspiracy to commit a legal act if it’s got some negative stigma your trying to avoid. For example students may conspire to lower the bar when graded on a bell curve.
Real, mundane conspiracies are numerous, and there are probably some outlandish ones, too. Every proven conspiracy must have started with someone's theory, if not an unprompted admission, so it's not always wrong to theorize. It's just silly to dedicate more than a little brain space*time to theories you've got no evidence for or hope of proving, conspiracy or otherwise.