Zero-day
Etymology
From the idea that the vendor has had "zero days" to address the flaw, because they only learn of it once it is already being exploited. Such vulnerabilities are traded on gray and black markets and prized by state hackers precisely because no patch yet exists.
noun · /ZEER-oh-DAY/
A software vulnerability unknown to the vendor, and the attacks that exploit it before any fix exists.
What it names
The most dangerous class of security hole — one defenders do not yet know about — explaining why some breaches are essentially impossible to prevent in advance.
Example
The breach used a zero-day, so even fully updated systems had no defense until an emergency patch shipped.
Related idea
A specific and severe kind of bug: a zero-day is a security flaw that attackers know and exploit before the maker even knows it exists.