Noun
(originally law) Used other than figuratively or idiomatically: see conspiracy, theory. 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.
(dismissive, derogatory) Hypothetical speculation that is commonly considered untrue or outlandish.
Source: en.wiktionary.orgConspiracy theory was tremendously popular, always and forever. People wanted such catastrophes to mean something more than mere individual madness, and so the hunt was on. Kim Stanley Robinson
You simply cannot invent any conspiracy theory so ridiculous and obviously satirical that some people somewhere don't already believe it. Robert Anton Wilson
The only other scenario that could explain everything, up to and including your own bizarre apperance, is a convoluted conspiracy theory involving the Russian Mafia and a crack team of plastic surgeons. Eoin Colfer
I don't believe in the moon landing conspiracy theory. I don't believe in Big Foot. Jerome Corsi
Human beings are pattern-seeking animals who will prefer even a bad theory or a conspiracy theory to no theory at all. Christopher Hitchens
The main thing that I learned about conspiracy theory is that conspiracy theorists actually believe in a conspiracy because that is more comforting. The truth of the world is that it is chaotic. Alan Moore