Define 'zero-day exploit'.

A zero-day exploit refers to a security vulnerability that is not yet known to the software vendor or the public, which attackers exploit right after discovering it. The term "zero-day" signifies that the existence of the vulnerability is known for zero days; in other words, the developers have not had any time to create a patch or security update to address the issue. This is critical in the realm of cybersecurity because once a zero-day vulnerability is exploited, there is typically no immediate defense against it, leaving systems exposed until a fix is developed and deployed.

The timely nature of such exploits makes them particularly dangerous, as they can lead to significant breaches before any mitigation strategies can be implemented. Understanding zero-day exploits emphasizes the importance of proactive security measures and continuous monitoring of systems to identify and potentially mitigate unknown vulnerabilities before they can be used maliciously.