Zero-Day
Also written as 0-day, Zero-Day Exploit, Zero-Day Vulnerability
A flaw being exploited before the vendor has released a fix, so on the day it becomes public there is no patch to apply.
Think of it like
A lock flaw the burglars know about before the locksmith does. Nothing to install, so you have to change how you guard the door.
Junior or senior?
Heavily overclaimed. Many people have responded to a zero-day; very few have found one.
Senior sounds like
Can describe what their organisation did in the window before a patch existed.
Ask them
“Talk me through a zero-day your team responded to. What did you do before a patch was available?”
Sounds like real experience
Describes the interim response — blocking a route, disabling a feature, extra monitoring — and who decided to accept the risk that was left.
Probe further if
Claims to have handled zero-days but describes only applying vendor patches, which is ordinary patching rather than zero-day response.