When someone detects that a software or hardware contains a potential security issue, the vendor is notified appropriately in order to take action. Software companies will fix the code and distribute a patch or software update. However, if someone else has detected a vulnerability and does not disclose the information to a vendor or does not patch the problem, it can turn dangerous
What is a Zero-Day Vulnerability Definition?
A zero-day vulnerability is defined as a software security flaw that has not been disclosed or discovered by vendors or developers. It can also be a vulnerability that has been disclosed, but may not have been patched yet. The term “zero-day” is used since the vendor has known about the vulnerability for zero days, thus it has no fix.
https://pacgenesis.com/what-does-zero-day-vulnerability-mean/