Definitions from The American Heritage Dictionary of the English Language, 5th Edition.
noun: An agreement to perform together an illegal, wrongful, or subversive act.
noun: A group of conspirators.
noun: Law An agreement between two or more persons to commit a crime or accomplish a legal purpose through illegal action.
noun A joining or acting together, as if by sinister design.
Further Information:
Conspiracy:
Definitions from The American Heritage Dictionary of the English Language, 5th Edition.
https://www.wordnik.com/words/conspiracy