What is the definition of "Zero-day?"

I've always used it to describe newly discovered vulnerabilities and exploits that are developing situations (such as Print Nightmare in the first few months after its discovery). However, I got pulled aside by our data governance officer who told me that it refers to known vulnerabilities that have no fix and/or will not have a patch released either due to the age of the product it affects or the nature of the vulnerability.

I did what any self-respecting IT person …

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