What term describes an agreement to perform an illegal act?

The term that describes an agreement to perform an illegal act is collusion. Collusion typically refers to a secret agreement or cooperation between parties to achieve a deceitful or unlawful purpose. In legal contexts, collusion often involves two or more parties conspiring to commit fraud, evade the law, or manipulate outcomes, such as in legal proceedings or market practices.

The other options relate to different concepts: treason involves betrayal of one's country, incitement refers to encouraging or urging someone to commit a crime, and assault is a physical attack or threat of violence against another person. Each of these terms addresses separate legal issues, whereas collusion specifically pertains to the agreement itself to engage in unlawful activity. Thus, this term accurately captures the essence of the question regarding unlawful agreements.