zero-day
adjective
ze·ro-day
ˈzir-(ˌ)ō-ˈdā
ˈzē-(ˌ)rō-
computing
: of, relating to, or being a vulnerability (as in a computer or computer system) that is discovered and exploited (as by cybercriminals) before it is known to or addressed by the maker or vendor
Thus, in the parlance of the trade, these vulnerabilities are known as "zero-day exploits," because it has been zero days since they have been uncovered and fixed.—James Bamford
There's a zero-day vulnerability in RealPlayer and Internet Explorer that allows malicious code to be installed …—Mark Hopkins
… most of what's on the black market doesn't have that zero-day freshness. Criminals typically target older vulnerabilities, for which fixes have already been released …—Lev Grossman
Cite this Entry
“Zero-day.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/zero-day. Accessed 5 Aug. 2026.
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