Meself, Wikipedia defines "conspiracy theory" as "an explanation of an event or situation that invokes an unwarranted conspiracy, generally one involving an illegal or harmful act carried out by government or other powerful actors. Conspiracy theories often produce hypotheses that contradict the prevailing understanding of history or simple facts. The term is often a derogatory one."

In other words, a conspiracy theory is a preposterous story of how some unseen force is taking us to hell in a handbasket. It does seem that in Internet forums, the term is being expanded to mean just about any explanation of things that the speaker thinks is stupid.

-Joe-