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The concept of "zero day" has two main meanings, depending on the context:
Zero-Day Vulnerability: As explained earlier, a "zero-day vulnerability" refers to a security flaw in software, hardware or any other system that is discovered and exploited by malicious actors before the developer of the system is aware of its existence . It's called "zero-day" because it gives attackers a chance to attack before some days (or time) have passed since the developer became aware of the vulnerability.
Zero-Day Attack: A "zero-day attack" refers to an attack that exploits a zero-day vulnerability. This is an attack that occurs before some time has elapsed since the discovery of the vulnerability, so the developer has not yet been able to release patches or fixes to address the vulnerability.
Basically, "zero day" in the context of cybersecurity refers to the fact that there is no delay between the discovery of a vulnerability and its exploitation by malicious parties. This gives attackers a temporary advantage as defenders have not had time to prepare or protect against the attack. It is a term that emphasizes the acute threat and urgency of such attacks.