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
James BamfordThere's a zero-day vulnerability in RealPlayer and Internet Explorer that allows malicious code to be installed …—Mark Hopkins
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
Lev GrossmanExamples of zero-day in a Sentence
Recent Examples on the Web
Anthropic has said its current Opus models are capable of autonomously identifying zero-day vulnerabilities in software.
—Beatrice Nolan, Fortune, 31 Mar. 2026
From simple oversights to novel prompt injection attacks, there’s no telling what the next zero-day vulnerability will be, especially in the new agentic world that Ashley and Shimel say has already arrived.
—Justyn Newman, PC Magazine, 27 Mar. 2026
Notably, Peter Williams, an executive of US government contractor Trenchant, was sentenced this month to seven years in prison for selling hacking tools to the Russian zero-day broker Operation Zero from 2022 to 2025.
—Andy Greenberg, Wired News, 3 Mar. 2026
Apple has released a major security update to fix a zero-day vulnerability, which is a security hole that hackers discover and exploit before the company has a chance to fix it.
—Kurt Knutsson, FOXNews.com, 17 Feb. 2026
See All Example Sentences for zero-day
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Beatrice Nolan, Fortune, 31 Mar. 2026
From simple oversights to novel prompt injection attacks, there’s no telling what the next zero-day vulnerability will be, especially in the new agentic world that Ashley and Shimel say has already arrived.
—Justyn Newman, PC Magazine, 27 Mar. 2026
Notably, Peter Williams, an executive of US government contractor Trenchant, was sentenced this month to seven years in prison for selling hacking tools to the Russian zero-day broker Operation Zero from 2022 to 2025.
—Andy Greenberg, Wired News, 3 Mar. 2026
Apple has released a major security update to fix a zero-day vulnerability, which is a security hole that hackers discover and exploit before the company has a chance to fix it.
—Kurt Knutsson, FOXNews.com, 17 Feb. 2026
See All Example Sentences for zero-day
Cite this Entry
“Zero-day.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/zero-day. Accessed 2 Apr. 2026.
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