What is Conspiracy?

Legal Definition
In criminal law, a conspiracy is an agreement between two or more persons to commit a crime at some time in the future. Criminal law in some countries or for some conspiracies may require that at least one overt act must also have been undertaken in furtherance of that agreement, to constitute an offense. There is no limit on the number participating in the conspiracy and, in most countries, no requirement that any steps have been taken to put the plan into effect (compare attempts which require proximity to the full offence). For the purposes of concurrence, the actus reus is a continuing one and parties may join the plot later and incur joint liability and conspiracy can be charged where the co-conspirators have been acquitted or cannot be traced. Finally, repentance by one or more parties does not affect liability – unless, in some cases, it occurs before the parties have committed overt acts – but may reduce their sentence.
-- Wikipedia
Legal Definition
An agreement between two or more people to commit an illegal act, along with an intent to achieve the agreement's goal. Most U.S. jurisdictions also require an overt act toward furthering the agreement. An overt act is a statutory requirement, not a constitutional one. See Whitfield v. United States, 453 U.S. 209 (2005). The illegal act is the conspiracy's "target offence."

Conspiracy generally carries a penalty on its own. In addition, conspiracies allow for derivative liability where conspirators can also be punished for the illegal acts carried out by other members, even if they were not directly involved. Thus, where one or more members of the conspiracy committed illegal acts to further the conspiracy's goals, all members of the conspiracy may be held accountable for those acts.

Where no one has actually committed a criminal act, the punishment varies. Some conspiracy statutes assign the same punishment for conspiracy as for the target offence. Others impose lesser penalties.

Conspiracy applies to both civil and criminal offenses. For example, you may conspire to commit murder, or conspire to commit fraud.

See Inchoate Offenses
Legal Definition
Crim. law, torts. An agreement between two or more persons to do an unlawful act, or an act which may become by the combination injurious to others. Formerly this offence was much more circumscribed in its meaning than it is now. Lord Coke describes it as "a consultation or agreement between two or more to appeal or indict an innocent person falsely and maliciously, whom accordingly they cause to be indicted or appealed and afterwards the party is acquitted by the verdict of twelve men."

2. The crime of conspiracy, according to its modern interpretation, may be of two kinds, Damely, conspiracies against the public, or such as endanger the public health, violate public morals, insult public justice, destroy the public peace, or affect public trade or business. See 3 Burr. 1321.

3. To remedy these evils the guilty persons may be indicted in the name of the commonwealth. Conspiracies against individuals are such as have a tendency to injure them in their persons, reputation, or property. The remedy in these cases is either by indictment or by a civil action.

4. In order to reader the offence complete, there is no occasion that any act should be done in pursuance of the unlawful agreement entered into between the parties, or that any one should have been defrauded or injured by it. The conspiracy is the gist of the crane. 2 Mass. R. 337; Id. 538 6 Mass. R. 74; 3 S. & R. 220 4 Wend. R. 259; Halst. R. 293 2 Stew. Rep. 360; 5 Harr. & John. 317 8 S. & R. 420. But see 10 Verm. 353.

5. By the laws of the United State's, St. 1825, c. 76, 23, 3 Story's L. U. S., 2006, a wilful and corrupt conspiracy to cast away, burn or otherwise destroy any ship or vessel. with intent to injure any underwriter thereon, or the goods on board thereof, or any lender of money on such vessel, on bottomry or respondentia, is, by the laws of the United States, made felony, and the offender punishable by fine not exceeding ten thousand dollars, and by imprisonment and confinement at hard labor, not exceeding ten years.

6. By the Revised Statutes of New York, vol. 2, p. 691, 692, it is enacted, that if any two or more persons shall conspire, either, 1. To commit any offence; or, 2. Falsely and maliciously to indict another for any offence; or, 3. Falsely to move or maintain any suit; or, 4. To cheat and defraud any person of any property, by any means which are in themselves criminal; or, 5. To cheat and defraud any person of any property, by means which, if executed, would amount to a cheat, or to obtaining property by false pretences; or, 6. To commit any act injurious to the public health, to public morals, or to trade and commerce, or for the perversion or obstruction of justice, or the due administration of the laws; they shall be deemed guilty of a misdemeanor. No other conspiracies are there punishable criminally. And no agreement, except to commit a felony upon the person of another, or to commit arson or burglary, shall be deemed a conspiracy, unless some act besides such agreement be done to effect the object thereof, by one or more of the parties to such agreement.

7. When a felony has been committed in pursuance of a conspiracy, the latter, which is only a misdemeanor, is merged in the former; but when a misdemeanor only has been committed in pursuance of such conspiracy, the two crimes being of equal degree, there can be no legal technical merger. 4 Wend. R. 265. Vide 1 Hawk. 444 to 454; 3 Chit. Cr. Law, 1138 to 1193 3 Inst. 143 Com. Dig. Justices of the Peace, B 107; Burn's Justice, Conspiracy; Williams' Justice, Conspiracy; 4 Chit. Blacks. 92; Dick. Justice Conspiracy, Bac. Ab. Actions on the Case, G 2 Russ. on Cr. 553 to 574 2 Mass. 329 Id. 536 5 Mass. 106 2 D R. 205; Whart. Dig. Conspiracy; 3 Serg. & Rawle, 220; 7 Serg. & Rawle, 469 4 Halst. R. 293; 5 Harr. & Johns. 317 4 Wend. 229; 2 Stew. R. 360;1 Saund. 230, u. 4. For the French law, see Merl. Rep. mot Conspiration Code Penal, art. 89.
-- Bouviers Law Dictionary
Legal Definition
In criminal law. A combination or confederacy between two or more persons formed for the purpose of committing, by their joint efforts, some unlawful or criminal act or some act which is innocent in itself, but becomes unlawful when done by the concerted action of the conspirators or for the purpose of using criminal or unlawful means to the commission of an act not in itself unlawful. Pettibone v. In S., 148 U. S. 107, 13 Sup. Ct. 542, 37 L. Ed. 419; State v. Slutz, 106 La. 182, 30 South. 298; Wright v. In S., 108 Fed. 805, 48 C. C. A. 37; U. S. v. Benson, 70 Fed. 591, 17 C. C. A. 293; Girdner v. Walker, 1 Heishi (Tenn.) 180; Boutwell v. Man, 71 Vt 1, 42 Atl. 607, 43 In In A. 803, 76 Am. St. Rep. 746; U. S. v. Weber (C. C.) 114 Fed. 950; Comm. v. Hunt, 4 Mete. (Mass.) Ill, 38 Am. Dec. 346; Brdman v. Mitchell, 207 Pa. 79, 56 Atl. 327, 63 L. R. A. 534, 99 Am. St. Rep. 783; Standard Oll Co. v. Doyle, 118 Ky. 662, 82 S. W. 271, 111 Am. St. Rep. 331.

Conspiracy is a consultation or agreement between two or more persons, either falsely to accuse another of a crime punishable by law; or wrongfully to injure or prejudice a third person, or any body of men, in any manner; or to commit any offence punishable by law; or to do any act with intent to prevent the course of justice; or to effect a legal purpose with a corrupt intent, or by improper means. Hawk. P. O. c. 72, § 2; Archb. Crim. Pi. 390, adding also combinations by journeymen to raise wages. State v. Murphy, 6 Ala. 765, 41 Am. Dec. 79.

Civil and criminal. The term "civil" is used to designate a conspiracy which will furnish ground for a civil action, as where, in carrying out the design of the conspirators, overt acts are done causing legal damage, the person injured has a right of action. It is said that the gist of civil conspiracy is the injury or damage. While criminal conspiracy does not require such overt acts, yet, so far as the rights and remedies are concerned, all criminal conspiracies are embraced within the civil conspiracies. Brown v. Pharmacy Co.., 115 Ga. 429, 41 S. E. 553, 57 In R. A. 547, 90 Am. SL Ren. 126.
-- Black's Law Dictionary
Legal Definition
A combination of two or more persons to procure an unlawful object, or to procure a lawful object by unlawful means. See 159 Pa. St. 420, 39 Am. St. Rep. 686, 23 L. R. A. 135, 28 Atl. 190.
-- Ballentine's Law Dictionary