zero-day
adj.— «0day is the list of vulnerabilities and exploits which have not been publicly acknowledged (SecurityFocus) or patched (vendors). No one knows about them. Since they’re not public knowledge, no one shares them.» —“A peek at script kiddie culture” by Andrew D. Kirch NewsForge Mar. 5, 2004. (source: Double-Tongued Dictionary)
Further reading
What if, instead of being an inanimate object, a dictionary were alive? That’s the idea behind a lavishly illustrated new children’s book called The Dictionary Story (Bookshop|Amazon) by Oliver Jeffers and Sam Winston. This is part of a...