The terms "conspiracy theorist" and "conspiracy nut" are used frequently to discredit a perceived adversary using emotional, rather than logical, appeals. It's important for the sake of true argument that we define the term "conspiracy" and use it appropriately, not as an ad hominem attack on someone whose point of view we don't share.
According to my Webster's Unabridged Dictionary, the word "conspiracy" derives from the Latin "conspirare" which means literally "to breath together" in the sense of agreeing to commit a crime. The primary definition is "planning and acting together secretly, especially for a harmful or unlawful purpose, such as murder or treason."
It was in this sense that Mark Twain astutely observed, "A conspiracy is nothing but a secret agreement of a number of men for the pursuance of policies which they dare not admit in public."
Conspiracies are common. If they weren't, police stations would not need conspiracy units to investigate and prosecute crimes such as "conspiracy to import cocaine" or any other collusion on the part of two or more people to subvert the law.
Unfortunately, too many people smugly chide "conspiracy theories" as if they imagine that such a derisive characterization reflects superior intellect--whether or not they know anything about the issue in question. It's a pitiful display of ego inflation and intellectual dishonesty, yet it appears to be a common approach preferred by those short on information and critical thinking skills.
Here are a few examples of past "conspiracy theories" that were commonly derided at the time of their occurrence but were later determined to be credible:
1933 Business Plot: Smedley Butler, a decorated United States Marine Corps major general who wrote a book called War is a Racket, testified before a congressional committee that a group of powerful industrialists, who had tried to recruit him, were planning to form a fascist veterans' group that intended to assassinate Franklin Roosevelt and overthrow the government in a coup. While news media at the time belittled Butler and called the affair a hoax, the congressional committee determined that Butler's allegations were credible although no-one was prosecuted.
Operation Paperclip: After "winning" WW2, the US imported hundreds of Nazis and their families through "Operation Paperclip," so-named because ID photos were clipped to paper dossiers. It was set up by an agency within the Office of Strategic Services, predecessor of the CIA. Along with creating false identities and political biographies, Paperclip operatives expunged or altered Nazi records and other criminal histories in order to illegally circumvent President Truman's edict that prohibited Nazis from obtaining security clearances. Thus, high-level Nazis waltzed into sensitive positions of authority and secrecy in the US military-industrial establishment, including the National Aeronautics and Space Administration (NASA), major corporations, and universities. These Germans were conveniently referred to as "former Nazis," but "former" was commonly just a euphemism for "active" and "ardent."
Consider the irony of the United States' moon mission. In order to successfully land men on the lunar surface and return them to Earth, the US depended almost exclusively on Nazis. A notable example was rocket scientist Wernher von Braun, a member of the Allgemeine SS, who would eventually lead the US space program. Von Braun had exploited concentration camp labor in Germany to build V-2 rockets at Peenemunde, and German aviation doctors' gruesome and often fatal experiments at Dachau and other prisons afforded information that would help keep American astronauts alive in space.
While many Americans prefer to call this a conspiracy theory, the United States defeated the Nazi organization in Germany only to transplant that ideology directly into the US after the war, and not just among members of the lay population but, more significantly, among members of the very "military-industrial complex" that President Eisenhower (a five-star general during World War Two) had presciently warned the nation about in his 1961 message of leave-taking and farewell.
Operation Northwoods: Declassified documents revealed that in 1962 the CIA was planning to execute false flag terrorist attacks, such as killing random American citizens and blowing up civilian targets, including a ship and US airliner, in order to blame Castro and justify invading Cuba.
Gulf of Tonkin: President Lyndon Johnson used a contrived version of this 1964 event to justify escalation of the Vietnam War. It was claimed that Vietnamese gunboats had fired on the USS Maddox. It never happened--or at best was grossly distorted and overblown--yet the story served to prompt Congress to pass the Gulf of Tonkin Resolution, which provided the public justification Johnson needed to attack North Vietnam. This led to the deaths of about two million Vietnamese people and fifty thousand Americans.
MK-ULTRA: As its code name suggests, MK-ULTRA was a mind control program run by the Office of Scientific Intelligence for the ostensible purpose of discovering ways to glean information from Communist spies although its applications were undoubtedly more far-reaching. It employed various methodologies including sensory deprivation and isolation, sexual abuse, and the administration of powerful psychotropic drugs such as LSD to unwitting subjects, including military personnel, prisoners, and college students. Many of them suffered serious consequences. One biochemist, Frank Olson, who was secretly slipped a strong dose of LSD at a staff party, suffered a severe psychotic break and died when, for whatever reason, he plummeted from his apartment window to the pavement below. Such revelations came to light in 1975 during hearings by the congressional Church Committee (Select Committee to Study Governmental Operations with Respect to Intelligence Activities) and the presidential Rockefeller Commission. These investigations were hindered by CIA Director Richard Helms who in 1973 had ordered the MK-ULTRA files destroyed.
Operation Mockingbird: This was a CIA media control program exposed by the Church Committee in 1975. It revealed the CIA's efforts from the 1950s through the 1970s to pay well-known foreign and domestic journalists from "reputable" media agencies such as the Washington Post, Time Magazine, Newsweek, the Miami Herald, the New York Times, the New York Herald Tribune, Miami News, and CBS, among others, to publish CIA propaganda, manipulating the news by planting stories in domestic and foreign news outlets. During the hearings, Senator Church asked an agency representative, "Do you have any people paid by the CIA who are working for television networks?" The speaker eyed his lawyer then replied, "This I think gets into the details, Mr. Chairman, that I'd like to get into in executive session." In other words, he didn't want to admit the truth publicly. He gave the same response when asked if the CIA planted stories with the major wire services United Press International (UPI) and the Associated Press (AP). According to Alex Constantine's book, Mockingbird: The Subversion of The Free Press by the CIA, in the 1950s "some 3,000 salaried and contract CIA employees were eventually engaged in propaganda efforts." I'm curious to know what the estimate would be today.