Which I thought was wrong, but even Wikipedia agrees with you:

"A zero-day (also known as 0-day) vulnerability is a computer-software vulnerability that is unknown to those who should be interested in mitigating the vulnerability (including the vendor of the target software)."

Personally I have always used it as "there is not mitigation / patch available". Thanks for pointing me in the proper direction.

Edit: I am old and stubborn, I still use 'hacker' as a compliment.