What is the defining characteristic of a 'zero-day exploit' in cybersecurity?

The defining characteristic of a 'zero-day exploit' is that it targets a vulnerability in software or hardware that is unknown to the vendor or developer and for which no patch or fix is available. This means that attackers can exploit the vulnerability before the developers have a chance to address it, making zero-day exploits particularly dangerous. The term 'zero-day' refers to the fact that the developers have zero days to fix the vulnerability once it's being actively exploited. Because there is no defense available, these exploits often lead to significant security breaches and data compromises. Detecting and preventing zero-day exploits requires advanced security measures and proactive threat hunting.