What defines a zero-day vulnerability?

A zero-day vulnerability refers to a security flaw that is exploited by attackers before the vendor has had a chance to release a patch or fix for it. This means that the vulnerabilities are 'zero days' old, indicating that the software developers have had no time to address the issue since it was discovered. The essence of a zero-day condition lies in the fact that the existence of the vulnerability is unknown to the software developer or vendor, making any attack utilizing this vulnerability particularly dangerous and potentially devastating, as there are no mitigative measures available to users or systems. This window of exposure can result in significant security breaches, as malicious actors can exploit the vulnerability without any immediate recourse for the impacted system administrators or users.