Explain what it means along with your statement. Maybe I have the wrong definition too.
If a security bug is exploited in the wild, it's an n-day if it's been first exploited n days after the publication of the bug, and a zero-day if it's been exploited before or on the day of the publication.
When a bug is not yet exploited in the wild, it's just a discovery of a bug, not a zero-day.
Originally a zero-day exploit was one that was found by crackers on the first day of release of a software product. Like finding a licence crack for a new Microsoft program on the day it went on sale.
There used to be fierce competition to find such an exploit within those 24 hours, and great kudos for those who did.
Nowadays a zero-day can apparently be found years after release, which makes no sense.
> [Z]ero-day specifically compares when the white hats (vendors, system owners) and the black hats learn about the existence of a vulnerability. If white hats learn that a vulnerability exists by being subject to an in-the-wild black hat exploit of it, then it's a true zero-day.
And, again, you need to be aware that the vulnerability is the flaw or defect in the software or system (e.g., buffer overrun), and the exploit is the specific methodology that takes advantage of it (e.g., worm, malicious web request from a botnet, etc.).
Some people differentiate between a zero day vulnerability and a zero day exploit. I don't really find that is common anymore, and essentially everyone using it means zero-day exploit.