What is the term for a secret agreement for illegal or deceitful purposes?

The term that best describes a secret agreement for illegal or deceitful purposes is collusion. Collusion involves two or more parties who secretly cooperate for a fraudulent or deceitful purpose, typically to create an unfair advantage or to circumvent legal obligations. This activity is usually aimed at misleading others, which aligns with the concept of deceit.

While conspiracy could also relate to secret agreements with illegal intent, it typically involves a broader range of actions and an agreement to commit a crime, making it distinct from the specific focus of collusion on cooperation among parties. Fraud generally refers to wrongful or criminal deception intended to result in financial or personal gain, but it doesn’t inherently require the element of a secret agreement. The term conspiracy theory often refers to beliefs about secret plots, but it lacks the legal framework surrounding illicit agreements.

Therefore, collusion appropriately captures the essence of a secretive arrangement aimed at achieving illicit goals, marking it as the correct choice in this context.