A Zero Day or Zero-Day vulnerability refers to a software security flaw that is known to the software vendor but doesn't have a patch in place to fix the vulnerability. It is called 'Zero Day' because the developers have 'zero days' to fix the problem that has just been exposed — and perhaps already exploited by hackers.
The term also refers to attacks that are launched before the vulnerability is publicly known. In the world of software security, Zero Day attacks are a serious threat because they take advantage of vulnerabilities in software that are unknown to the software maker or user.
Therefore, protection against such threats often relies on proactive cybersecurity practices rather than reactive measures.