Conspiracy is ably well defined in each jurisdicition's criminal law. In Canada, the definition is:
[quote="The Criminal Code R.S.C. 1985, c. C-46]21(2) Where two or more persons form an intention in common to carry out an unlawful purpose and to assist each other therein and any one of them, in carrying out the common purpose, commits an offence, each of them who knew or ought to have known that the commission of the offence would be a probable consequence of carrying out the common purpose is a party to that offence.[/quote]
The necessary elements of conspiracy are:
1) two or more persons
2) formation of a common intent
3) to carry out an unlawful purpose and assist each other
4) any one of them commits an offence
5) while carrying out the common purpose.
Secrecy is not a necessary element of conspiracy, but it does require a member of the conspiracy to attempt to carry out the purpose and to commit an offence as a result.
If all you do, as a member of a group, is agree, "let's rob a bank!" and then walk into the bank together, no offence has occurred--and not conspiracy. But if one of you then creates a disturbance, then conspiracy is crystallized and all of you have committed the offence.
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--James