A zero-day threat is an attack that takes advantage of a security vulnerability that does not have a fix in place. It is referred to as a "zero-day" threat because once the flaw is eventually discovered, the developer or organization has "zero days" to then come up with a solution. A zero-day threat can compromise a system or network by exploiting the unknown vulnerability, and can cause data loss, unauthorized access, or other damages. Zero-day threats are difficult to detect and prevent, and require advanced security solutions and practices to mitigate them. Reference:
Palo Alto Networks Certified Cybersecurity Entry-level Technician (PCCET) Zero-day (computing) - Wikipedia What is a zero-day exploit? | Zero-day threats | Cloudflare