An agreement between two or more persons to commit an illegal or unlawful act, or to achieve a legal act but by illegal or unlawful means.
R. v. Cotroni, Canada's Supreme Court wrote.
"The word conspire derives from two Latin words, "con" and "spirare", meaning "to breathe together".
"To conspire is to agree.
"The essence of criminal conspiracy is proof of agreement. On the charge of conspiracy the agreement itself is the gist of the offence."
Or, from Boots v Grundy (England, 1900):
"A conspiracy exists when two or more combine to do an unlawful act or to do a lawful act by unlawful means. No conspiracy is ... known to the law which has not for its object the accomplishment of an unlawful act (not necessarily a criminal act) or which does not involve the use of unlawful means".
Those forming the conspiracy are called conspirators.
Conspiracy can form the basis for an action in tort as well as constituting a crime.
Quoting from the 1983 decision of the Supreme Court of Canada in Canada Cement Lafarge Ltd., this is the succinct summary as to tort law, where conspiracy is often referred to and distinguished as a civil conspiracy (see Legal Definition of Civil Conspiracy), and tendered in Alford v Canada:
"There are two types of actionable conspiracy in Canada: (1) Where the predominant purpose of the defendants' conduct is to injure the plaintiffs, whether the means used by the defendants are lawful or unlawful; and (2) where the defendants conduct is unlawful and is directed towards the plaintiffs (alone or together with others) and the defendant should have known that injury to the plaintiffs is likely to and does result.To be complete, the tort of conspiracy requires not only a conspiratorial agreement, but also proof that overt acts have caused damage to the plaintiffs."
In criminal law, from R v Lam:
"The crime of conspiracy is ... an agreement between two or more persons to commit an indictable offence. The actual commission of the indictable offence which is the subject of the agreement may be relied upon as evidence of the conspiratorial agreement, but it is not an element of the offence of conspiracy. The offence is complete once the agreement has been entered even though the indictable offence which is the subject of the agreement is never actually committed. It is also not necessary in order for an accused to be convicted of conspiracy for the agreement to have called for the accused to himself commit the indictable offence which is the subject of the conspiratorial agreement."
In Canada, the crime of conspiracy is set out at §465 of the Criminal Code which includes, inter alia:
"(E)very one who conspires with any one to commit an indictable offence ... is guilty of an indictable offence and liable to the same punishment as that to which an accused who is guilty of that offence would, on conviction, be liable."
Note, also, the oft-quoted words of Justice Jackson of the United States Supreme Court in Krulewitch where he spoke of the crime of conspiracy as that:
"... elastic, sprawling and pervasive offense."
- Alford v Canada 31 B.C.L.R. (3d) 228 (1997)
- Boots v Grundy, 82 Law Times 769 (England, 1900)
- Canada Cement Lafarge Ltd. v. BC Lightweight Aggregate Ltd.,  1 SCR 452
- Krulewitch v. United States, 336 US 440 (1948)
- R. v. Cotroni (1979), 2 SCR 256
- R v Lam, 2005 ABQB 849