It refers to your many days software is available for, with zero implying it is not yet out so you couldn't have installed a new version and that's what makes it a risky bug

The term has long watered-down to mean any vulnerability (since it was always a zero-day at some point before the patch release, I guess is those people's logic? idk). Fear inflation and shoehorning seems to happen to any type of scary/scarier/scariest attack term. Might be easiest not to put too much thought into media headlines containing 0day, hacker, crypto, AI, etc. Recently saw non-R RCEs and supply chain attacks not being about anyone's supply chain copied happily onto HN

Edit: fwiw, I'm not the downvoter

It's original meaning was days since software release, without any security connotation attached. It came from the warez scene, where groups competed to crack software and make it available to the scene earlier and earlier. A week after general release, three days, same-day. The ultimate was 0-day software, software which was not yet available to the general public.

In a security context, it has come to mean days since a mitigation was released. Prior to disclosure or mitigation, all vulnerabilities are "0-day", which may be for weeks, months, or years.

It's not really an inflation of the term, just a shifting of context. "Days since software was released" -> "Days since a mitigation for a given vulnerability was released".

I think the implication in this specific context is that malicious people were exploiting the vuln in the wild prior to the fix being released