A "Zero-day" vulnerability indicates a security flaw in software that is exploited by attackers before the vendor is aware or has a chance to release a fix. These vulnerabilities are particularly dangerous as they can be used in cyberattacks, putting systems and networks at risk. The term "zero-day" refers to the fact that the developers have had zero days to address or patch the identified flaw, leaving users vulnerable. This highlights the importance of ongoing security vigilance and timely updates in software management.