A Zero-day (or 0-day) vulnerability is a vulnerability that is unknown to end users, software vendors, security experts or anyone else other than the attacker who discovered the vulnerability. There is then no specific measure to prevent them as they are simply unknown. You can try to mitigate the blast radius of a 0-day with these measures, but you can't prevent them entirely. Once 0-days are discovered they are generally patched quickly and updates from software vendors will be made available.
Wikipedia
A zero-day (also known as a 0-day) is a vulnerability or security hole in a computer system unknown to its owners, developers or anyone capable of mitigating it. Until the vulnerability is remedied, threat actors can exploit it in a zero-day exploit, or zero-day attack.The term "zero-day" originally referred to the number of days since a new piece of software was released to the public, so "zero-day software" was obtained by hacking into a developer's computer before release. Eventually the term was applied to the vulnerabilities that allowed this hacking, and to the number of days that the vendor has had to fix them. Vendors who discover the vulnerability may create patches or advise workarounds to mitigate it. Still users need to deploy the mitigation to eliminate the vulnerability in their systems. Zero-day attacks are severe threats.