A "Zero Day" refers to a security vulnerability in software that is unknown to the vendor or developer. This means that there is no available patch or fix to address the flaw, leaving systems exposed to potential attacks. Cybercriminals can exploit these vulnerabilities to gain unauthorized access or cause damage before the issue is publicly known. Once a Zero Day vulnerability is discovered, it can be sold on the black market or used in cyberattacks. Organizations often prioritize identifying and mitigating these risks to protect their systems. The term is commonly associated with cybersecurity and software development practices.