A zero-day (or 0-day) vulnerability is a software vulnerability that is discovered by attackers before the vendor has become aware of it. By definition, no patch exists for zero-day vulnerabilities and user systems have no defenses in place, making attacks highly likely to succeed. The name "zero-day" is designed to imply how long since the vulnerability was discovered. The term also indicates that system developers have had zero days to fix it.
Intrusion Protection
While the precise methods of a zero-day exploit can't be known in advance, a network intrusion protection system (NIPS) can monitor the firms' network for unusual activity.
The advantage of NIPS over a traditional antivirus only system is it does not rely on checking software against a known database of threats. This means it does not need updates or patches to learn about the latest attacks. NIPS works by monitoring the day-to-day patterns of network activity across the network.
When traffic or events far out of the ordinary are detected action can be taken to alert system administrators and lock down the firewall. Devices such as USB drives and mobile devices can all introduce threats to the network. They can often make it past the firewall because they are physically introduced to the system.
NIPS protects against threats introduced to the network from both external and internal sources.
Reference: https://cybriant.com/how-to-prevent-zero-day-attacks-in-5-steps/