[2] Procedures and proof requirements for conspiracy theory litigation as well as the definition of 'conspiracy' vary by jurisdiction (British, U.S., …) and body of law (criminal, civil, …).
In civil litigation, it can offer advantages relative to aiding-and-abetting or joint tortfeasor case theories.
[3] In popular usage, the term ‘conspiracy’ means a secret agreement of two or more persons usually to commit a bad act.
In a broad legal sense, it is an agreement to commit an unlawful act; in British and some American courts, lawful acts finish in an unlawful manner (in British parlance, a ‘conspiracy to injure’; in American, a ‘true conspiracy’) are also included.
Whereas in a criminal conspiracy, the substantive offense is a crime, in civil law, the wrong is most likely a recognized intentional tort.