In cybersecurity, what does "zero-day" refer to?

The term "zero-day" specifically refers to a previously unknown vulnerability in software or hardware that has not yet been patched or addressed by the vendor. This type of vulnerability is particularly dangerous because it can be exploited by attackers before the security patch is released, leaving systems vulnerable. The term "zero-day" highlights that there has been zero days of protection since the discovery of the vulnerability.

When a zero-day vulnerability is discovered, attackers can exploit it immediately, making it crucial for organizations to be vigilant and to have strong security practices in place. Because these vulnerabilities are unknown to the software developers, they do not have a fix available, thus the urgency and risk associated with them.

In contrast, other choices do not capture the essence of a zero-day. A day without incidents does not relate to vulnerabilities. A type of security update refers to a response to known issues, and a standard security protocol does not necessarily imply vulnerability but rather a method of practice in security measures. Therefore, the definition focused on an unknown and unpatched vulnerability best defines a "zero-day."