A "zero-day exploit" in cybersecurity refers to a vulnerability in software that is unknown to the vendor and has not yet been patched. Hackers can exploit this vulnerability to launch attacks before the software developer has the opportunity to release a fix, thus the term "zero-day." These exploits are particularly dangerous because there are no defenses available against them at the time of the attack. Organizations must proactively implement security measures and stay informed about security updates to protect against such threats, as they can lead to significant data breaches or system compromises.