con‧spi‧ra‧cy plural conspiracies [uncountable and countable]
a secret plan made by two or more people to do something that is harmful or illegal [↪ conspire]
conspiracy to do something
He was charged with conspiracy to commit criminal damage.
a conspiracy against the government
There were many conspiracy theories (=beliefs that something is the result of a conspiracy) surrounding Princess Diana's death.
an agreement not to talk about something, even though it should not be a secret:
There's often a conspiracy of silence surrounding bullying in schools.