What does the term "zero-day" refer to in cybersecurity?

The term "zero-day" in cybersecurity specifically refers to a recently discovered security vulnerability for which no patch or fix is available at the time of its discovery. This means that the vulnerability is "zero days old," indicating that it has just been identified, giving attackers an opportunity to exploit it before the developers can respond and issue a fix. Since there is no fix at the time, it is particularly dangerous because it leaves systems vulnerable to attacks.

In contrast, the other options relate to different concepts in cybersecurity. A discovered security vulnerability known for years does not match the zero-day definition because it has had time for developers to issue fixes and users to apply them. An outdated security patch refers to a patch that is no longer effective due to newer vulnerabilities emerging or because it does not address all security concerns; this also doesn't align with the zero-day definition. Lastly, a software development lifecycle refers to the phases of development that software goes through, which is unrelated to the specific urgency and exploitation potential of zero-day vulnerabilities.