A "zero-day vulnerability" refers to a security flaw in software that is known to the vendor but not yet released a fix or patch for. This term highlights the critical nature of the vulnerability since it can be exploited by malicious actors before users and developers are aware or able to protect their systems. Zero-day attacks can lead to severe consequences, including data breaches and system compromises, underscoring the importance of timely security updates and proactive measures in cybersecurity.