A "Zero-Day" refers to a security vulnerability in software that is unknown to the vendor or developer. This means that there are zero days of protection against potential exploitation. Cybercriminals can take advantage of these vulnerabilities to launch attacks before the software creators have a chance to fix the issue. Once a zero-day vulnerability is discovered, it can be sold on the black market or used in cyberattacks. Companies like Microsoft and Apple work continuously to identify and patch these vulnerabilities to protect users. The term highlights the importance of cybersecurity in today's digital landscape.