The word conspiracy comes from the Latin "con spire", which means "to breathe together". A conspiracy is any secretive or unlawful group of people assembled for a plan.
In the United States, most criminal laws regarding conspiracy require that an overt act in furtherance of the conspiracy be committed before a crime is said to have occurred. This overt act in and of itself need not be illegal. For example, if two or more persons plot to rob a bank, and one of these commits an overt act in furtherance of the plot, such as obtaining the schedule of security guards at the bank, or blueprints of the bank, the plotters may be charged and convicted of conpiracy to rob a bank.