The meanings of "conspiracy theory" according Wiktionary, my favorite dictionary:
> 1. A hypothesis alleging that the members of a coordinated group are, and/or were, secretly working together to commit illegal or wrongful actions including attempting to hide the existence of the group and its activities. In notable cases the hypothesis contradicts the mainstream explanation for historical or current events. [1960s]
The first sentence describes something that certainly does happen. Organized crime syndicates are conspiracies. If somebody tries to hire a hitman to kill their neighbor, they'll be charged with conspiracy to commit murder. Conspiracies are real things that sometimes happen.
The second sentence is where things get interesting. As far as I've been able to determine, this started with the assassination of JFK. Officially, he was killed by a single man who acted on his own and didn't conspire with anybody else. Any theory that posits the gunman was in cahoots with anybody else was a conspiracy theory in the original sense the term. In the aftermath of that, 'conspiracy theory' seemingly became mixed up with the premise of disagreeing with the mainstream explanation. But sometimes the mainstream narrative is a theory about people conspiring. The man who shot Lincoln conspired with others; that's the mainstream narrative in that case and I have no reason to doubt it.
It's all very tiresome.