A conspiracy is a secret agreement between two or more people to commit an unlawful or harmful act. Conspiracy theorists are people who believe that the government is secretly controlled by power brokers in flagrant violation of the constitution.
Conspiracy can also refer to the act of planning an unlawful or harmful act: Terrorists might be accused of organizing a conspiracy to overthrow the government. Conspiracy is ultimately from Latin cōnspīrāre "to agree or plot together, literally to breathe together." The corresponding English verb is conspire.