General Terms:
* Conspiracy: This is the most common and general term, encompassing any secret agreement to commit a crime.
* Collusion: This implies a secret agreement to deceive or cheat someone.
* Cabal: This suggests a small, secretive group working together for an illicit purpose.
* Plot: This implies a scheme or plan, often involving deception or violence.
More Specific Terms:
* Compact: This refers to a formal agreement, often written, for illegal activity.
* Pact: Similar to "compact", this emphasizes the binding nature of the agreement.
* Underworld deal: This refers to a secret agreement made within a criminal organization.
* Backroom deal: This suggests an agreement made in secret, often with illicit motives.
* Crooked deal: This emphasizes the illegal and unethical nature of the agreement.
The best word to use depends on the specific context and the nature of the illegal act. For example, "conspiracy" is appropriate for a broad range of criminal activities, while "underworld deal" is more specific to organized crime.