A conspiracy, also known as a plot, is a secret plan or agreement between people (called conspirers or conspirators) for an unlawful or harmful purpose, such as murder or treason, especially with political motivation, while keeping their agreement secret from the public or from other people affected by it. In a political sense, conspiracy refers to a group of people united in the goal of usurping, altering or overthrowing an established political power. Depending on the circumstances, a conspiracy may also be a crime, or a civil wrong. The term generally implies wrongdoing or illegality on the part of the conspirators, as people would not need to conspire to engage in activities that were lawful and ethical, or to which no one would object.

There are some coordinated activities that people engage in with secrecy that are not generally thought of as conspiracies. For example, intelligence agencies such as the American CIA and the British MI6 necessarily make plans in secret to spy on suspected enemies of their respective countries and the general populous of its home countries, but this kind of activity is generally not considered to be a conspiracy so long as their goal is to fulfill their official functions, and not something like improperly enriching themselves. Similarly, the coaches of competing sports teams routinely meet behind closed doors to plan game strategies and specific plays designed to defeat their opponents, but this activity is not considered a conspiracy because this is considered a legitimate part of the sport. Furthermore, a conspiracy must be engaged in knowingly. The continuation of social traditions that work to the advantage of certain groups and to the disadvantage of certain other groups, though possibly unethical, is not a conspiracy if participants in the practice are not carrying it forward for the purpose of perpetuating this advantage.

On the other hand, if the intent of carrying out a conspiracy exists, then there is a conspiracy even if the details are never agreed to aloud by the participants. CIA covert operations, for instance, are by their very nature hard to prove definitively but research into the agency's work, as well as revelations by former CIA employees, has suggested several cases where the agency tried to influence events. During the Cold War, the United States tried to covertly change other nations' governments 66 times, succeeding in 26 cases.

A "conspiracy theory" is a belief that a conspiracy has actually been decisive in producing a political event of which the theorists strongly disapprove. Political scientist Michael Barkun has described conspiracy theories as relying 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."