con·spir·a·cy
/kənˈspirəsē/
noun
noun: conspiracy; plural noun: conspiracies
1. a secret plan by a group to do something unlawful or harmful.
I'm pretty sure Osama and the rest of his terrorist buddies had a secret plan to do something unlawful and harmful.
conspiracy theory
n.
1. A theory seeking to explain a disputed case or matter as a plot by a secret group or alliance rather than an individual or isolated act.
2. the belief that the government or a covert organization is responsible for an event that is unusual or unexplained, esp when any such involvement is denied
I really love that second definition, because it proves the circular logic that always follows conspiracy theorists.
CTN: You were involved in the assassination of FDR.
M: What are you talking about, I had nothing to do with that!
CTN: See! You deny it, that proves you were involved!
M: But I was not even born yet!
CTN: Show me your birth certificate, I know it is a fake!
There is a reason I consider most who chase these theories to be mentally challenged. There is a deep level of paranoia involved with all of them in my experience.