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.
"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." R. v. Cotroni (1979), 2 SCR 256.
Or, from Boots v Grundy 82 Law Times 769 (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 acomplishment of an unlawful act (not necessarily a criminal act) or whch does not involve the use of unlawful means".
Those forming the conspiracy are called conspirators.
So, conspiracy is a tort as well as a crime.
Quoting from the 1983 decision of the Supreme Court of Canada in Canada Cement Lafarge Ltd. v. BC Lightweight Aggregate Ltd. (1 SCR 452), this is the succinct summary as to tort law, tendered in 1997 in Alford v Canada 31 B.C.L.R. (3d) 228:
"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 2005 ABQB 849, published at canlii.org/en/ab/abqb/doc/2005/2005abqb849/2005abqb849.html:
"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."