zero-day

adjective

ze·​ro-day ˈzir-(ˌ)ō-ˈdā How to pronounce zero-day (audio)
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

Examples of zero-day in a Sentence

Recent Examples on the Web
Examples are automatically compiled from online sources to show current usage. Read More Opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback.
Investigators found no evidence of zero-day exploits. Emil Sayegh, Forbes.com, 30 Aug. 2025 Bitdefender Total Security 2025 offers protection against viruses, worms, Trojans, ransomware, spyware, and zero-day exploits, ensuring that you're covered against any digital threats. PC Magazine, 22 Aug. 2025 The zero-day the group used is now being tracked as CVE-2025-8088. Dan Goodin, ArsTechnica, 12 Aug. 2025 Hackers are actively exploiting a new zero-day bug in Microsoft's SharePoint Server software. Kurt Knutsson, FOXNews.com, 3 Aug. 2025 See All Example Sentences for zero-day

Word History

First Known Use

2000, in the meaning defined above

Time Traveler
The first known use of zero-day was in 2000

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Cite this Entry

“Zero-day.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/zero-day. Accessed 9 Sep. 2025.

Last Updated: - Updated example sentences
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