Conspiracy:

1. A combination of men for an evil purpose; an agreement, between two or more persons, to commit a crime in concert, as treason; a plot.

2. A concurrence or general tendency, as of circumstances, to one event, as if by agreement.

3. (Law) An agreement, manifesting itself in words or deeds, by which two or more persons confederate to do an unlawful act, or to use unlawful to do an act which is lawful; confederacy.

From Webster's Revised Unabridged Dictionary, 1913.