Likelihood refers to the probability that a potential vulnerability could be exploited by a threat actor within a given time frame. Calculating likelihood involves evaluating how exposed the vulnerability is, the presence and capabilities of threat actors, the effectiveness of current controls, and the historical data of security incidents similar to the one being assessed. While options such as evaluating the impact of the threat and considering the cost of potential security controls are also parts of risk assessment, they do not directly relate to the determination of likelihood.