Hold the line! Let's make the term useful. An 0-day is an 0-day before public disclosure and for 24 hours after.
At one point in history a "0day" referred to a pirated release of software on/before it hit the shelves (and/or the crack that was released simultaneous to protected software aka "warez").
It was years later that a "0day" went from a copy protection removal/crack ("0day warez") to its more general modern usage in computer security.
See: https://www.google.com/books/edition/_/8ETRQhDytIsC?hl=en&gb...
Which was/is more relevant when commercial software is updated at most once ever year or two
But it appears no one can agree anymore, making the term useless.