The first time I heard it, it meant that it was a vuln present since the launch of that version unknown to the publisher, so meaning on day 0 it was possible to exploit it.

Then, when I was studying for a certification I learned they changed the definition to mean it was the number of days a vendor had to patch that vulnerability, and in some cases after being patched these vulnerabilities would get a name assigned to them.

And now, is more of a clickbait word for articles.