A zero day vulnerability in a software application is a vulnerability that is unknown to the developer of that software application. For example, if there is a vulnerability in your Windows 10 operating system and Microsoft is not aware of this vulnerability and this vulnerability can be used by hackers to cause mischief, it is a “zero day vulnerability”.
The hacking attack that takes place due to this “zero day vulnerability” is called a “zero day attack”.
A zero day vulnerability is mostly known to close hacking groups.
Mainstream companies, in case they come across a zero day vulnerability from another company, are expected to inform the company with the vulnerability so that the vulnerability can be patched. Then after a month, this vulnerability can be made public by the company that finds it.