What is the most appropriate term for a reward given to a government official after a decision has been made?

The most appropriate term for a reward given to a government official after a decision has been made is "illegal gratuity." This term specifically refers to a situation where a payment or benefit is given to a public official after they have completed a specific action or decision, indicating that the official was rewarded for their past actions rather than incentivized to take a particular action beforehand.

In contrast, bribery typically occurs before or in the midst of a decision-making process, where there is an intended influence on the official’s actions. Collusion refers to an agreement between parties to deceive or defraud others, rather than a reward to an official after a decision. Favoritism implies preferential treatment based on personal relationships rather than financial incentives. Thus, "illegal gratuity" is the correct term as it specifically captures the idea of a post-decision reward, aligning with the definition used in legal contexts regarding public corruption.