Zero Day
In plain terms
A vulnerability exploited before a fix is available.
Going deeper
A zero-day is a flaw unknown to the vendor (or unpatched) that attackers exploit, leaving 'zero days' to prepare a defense. They're especially dangerous.
Under the hood
Highly valued by attackers; defense relies on layered security and rapid response since no patch exists yet.
Examples
Unknown exploits, targeted attacks, Stuxnet, emergency patching, threat intel.
Think of it like...
Like a burglar exploiting a secret flaw in a lock the maker doesn't even know about yet.