In the risk assessment process, calculating the probability that a threat source will exploit an existing system vulnerability is known as likelihood analysis. This step involves evaluating how probable it is that the organization's vulnerabilities can be exploited by potential threats, considering various factors such as the nature of the vulnerability, the presence and capability of threat actors, and the effectiveness of current controls. Elizabeth's task of assessing the probability of exploitation is crucial for understanding the risk level associated with different vulnerabilities and for prioritizing risk mitigation efforts based on the likelihood of occurrence.