Nick Terry
Illuminator
And that's where it all falls apart, if we can't agree on a definition.
A conspiracy theory is an explanatory or speculative hypothesis suggesting that two or more persons or an organization have conspired to cause or to cover up, through secret planning and deliberate action, an event or situation typically regarded as illegal or harmful. Since the mid-1960s, the phrase has denoted explanations that invoke conspiracies without warrant, often producing hypotheses that contradict the prevailing understanding of historical events or simple facts.
https://en.wikipedia.org/wiki/Conspiracy_theory
This is the commonly-accepted meaning of "conspiracy theory", and stands in sharp contrast to proven conspiracy.
A conspiracy theory is an unproven claim of conspiracy. As in: unsubstantiated, lacking in evidence, made up, fabricated, fantasised, whatever.
A real conspiracy can be taken to criminal court and would result in a conviction. A conspiracy theory would get thrown out of civil court, like many Birther and 9/11 Truther lawsuits.
A real conspiracy becomes an accepted fact across all relevant humanities (eg history) and social science disciplines, internationally. A conspiracy theory fails to do this, and is not infrequently promoted by emeriti appealing to claims from disciplines in which they have no training (eg James Fetzer, philosopher, or David Ray Griffin, theologian, resorting to various forms of techno-babble about 9/11).
There is of course a continuum and there are exceptions. Unsolved mysteries, like the assassination of Olof Palme, would be one example; there are many theories about this, none of which have been proven.