A conspiracy theory is an explanation of an event or situation that invokes a conspiracy without warrant, 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 a derogatory one.
According to the political scientist Michael Barkun, conspiracy theories rely on the view that the universe is governed by design, and embody three principles: nothing happens by accident, nothing is as it seems, and everything is connected. Another common feature is that conspiracy theories evolve to incorporate whatever evidence exists against them, so that they become, as Barkun writes, a closed system that is unfalsifiable, and therefore "a matter of faith rather than proof".
People formulate conspiracy theories to explain, for example, power relations in social groups and the perceived existence of evil forces. Conspiracy theories have chiefly psychological or socio-political origins. Proposed psychological origins include projection; the personal need to explain "a significant event [with] a significant cause;" and the product of various kinds and stages of thought disorder, such as paranoid disposition, ranging in severity to diagnosable mental illnesses. Some people prefer socio-political explanations over the insecurity of encountering random, unpredictable, or otherwise inexplicable events. Some philosophers have argued that belief in conspiracy theories can be rational.
The Oxford English Dictionary defines conspiracy theory as "the theory that an event or phenomenon occurs as a result of a conspiracy between interested parties; spec. a belief that some covert but influential agency (typically political in motivation and oppressive in intent) is responsible for an unexplained event", and cites a 1909 article in The American Historical Review as the earliest usage example. As a neutral term, "conspiracy" is derived from Latin con- ("with, together") and spirare ("to breathe").
According to John Ayto's 20th century words, the phrase conspiracy theory was originally a neutral term and only acquired a pejorative connotation in the mid 1960s, implying that the advocate of the theory has a paranoid tendency to imagine the influence of some powerful, malicious, covert agency in events. According to Florida State University professor Lance deHaven-Smith's 2013 book Conspiracy Theory in America, the phrase conspiracy theory was deployed in the 1960s by the Central Intelligence Agency (CIA) to discredit John F. Kennedy assassination conspiracy theories. However, according to Robert Blaskiewicz, assistant professor of critical thinking at and skeptical activist, such claims have existed "since at least 1997", but due to having recently been promoted by deHaven-Smith, "conspiracy theorists have begun citing this work as an authority". Blaskiewicz researched the use of the term conspiracy theory and found that it has always been a disparaging term, having been used to describe "extreme hypothesis" and implausible speculation as long ago as 1870.