A zero-day vulnerability refers to a security flaw or weakness in a software application or system that is unknown to the vendor or developers, leaving it exposed to potential exploitation by attackers. The term "zero-day" implies that the vulnerability is discovered and exploited by hackers before the software vendor becomes aware of it, giving them zero days to address and patch the vulnerability. As a result, no official fixes or countermeasures are available to protect against such attacks.
Some examples of zero-day vulnerabilities are the following:
Stuxnet Worm in 2010
Petya Ransomware in 2016
Adobe Flash Player in 2015