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
Think of it as a zero-day exploit.
—Aidan McLaughlin, Vanity Fair, 16 Apr. 2026
To protect the Pixel modem from zero-day attacks, Google focused on the DNS parser.
—Scharon Harding, ArsTechnica, 13 Apr. 2026
The cutting-edge artificial intelligence business announced last week that its next-generation model, Mythos, has discovered zero-day vulnerabilities — previously unknown software bugs that can be used by hackers to compromise systems — in all major internet browsers and operating systems.
—Editorial Board, Washington Post, 12 Apr. 2026
In just the past few weeks, Anthropic says its Mythos Preview has identified thousands of zero-day vulnerabilities, many of which were critical and difficult to detect, including some in every major operating system and web browser.
—Beatrice Nolan, Fortune, 7 Apr. 2026
See All Example Sentences for zero-day
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.
Aidan McLaughlin, Vanity Fair, 16 Apr. 2026
To protect the Pixel modem from zero-day attacks, Google focused on the DNS parser.
—Scharon Harding, ArsTechnica, 13 Apr. 2026
The cutting-edge artificial intelligence business announced last week that its next-generation model, Mythos, has discovered zero-day vulnerabilities — previously unknown software bugs that can be used by hackers to compromise systems — in all major internet browsers and operating systems.
—Editorial Board, Washington Post, 12 Apr. 2026
In just the past few weeks, Anthropic says its Mythos Preview has identified thousands of zero-day vulnerabilities, many of which were critical and difficult to detect, including some in every major operating system and web browser.
—Beatrice Nolan, Fortune, 7 Apr. 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 1 May. 2026.
Copy CitationLove words? Need even more definitions?
Merriam-Webster unabridged
