Creative Speculation

The Oxford English Dictionary records the first use of the phrase "conspiracy theory" to a 1909 article in The American Historical Review. Other sources predate this use by nearly four decades to at least 1871, where it is used in The Journal of Mental Science reporting on a conference of the Fifth Quarterly Meeting of the Medico-Psychological Association (now the Royal College of Psychiatrists), held on January 27, 1870:

"The theory of Dr. Sankey as to the manner in which these injuries to the chest occurred in asylums deserved our careful attention. It was at least more plausible that the conspiracy theory of Mr. Charles Beade..."

Acquired derogatory meaning

Originally a neutral term, since the mid-1960s it has acquired a somewhat derogatory meaning, implying a paranoid tendency to see the influence of some malign covert agency in events. The term is often used to automatically dismiss claims that the critic deems ridiculous, misconceived, paranoid, unfounded, outlandish, or irrational. A conspiracy theory that is proven to be correct, such as the notion that United States President Richard Nixon and his aides conspired to cover up Watergate, is usually referred to as something else, such as investigative journalism or historical analysis.

http://en.wikipedia.org/wiki/Conspiracy_theory