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

Examples of zero-day in a Sentence

Recent Examples on the Web

And the trading community has mostly lost interest, having discovered zero-day options, 2x levered single-stock ETFs, prediction markets, sports betting and other new methods of speculation. — Josh Brown,sean Russo, CNBC, 6 Aug. 2026

In OpenAI's case the models found a previously unknown vulnerability, a zero-day, in a package registry proxy and used it to reach the internet. — Ashish Bhatia, Forbes.com, 1 Aug. 2026

Earlier this month, OpenAI said its security models exploited a zero-day vulnerability for use in breaking into the network of Hugging Face, a platform for open source machine-learning models and AI datasets. — Dan Goodin, ArsTechnica, 31 July 2026

None of the Claude models attempted to deliberately escape their test environment, unlike OpenAI's incident, in which its models exploited a previously unknown zero-day vulnerability in third-party software to access the internet. — James Peckham, PC Magazine, 31 July 2026

Word History

First Known Use

2000, in the meaning defined above

Rhymes for zero-day

• aaa
• alleyway
• alumnae
• anime
• anyway
• arete
• attache
• axillae
• baa
• beaujolais
• bouvier
• breakaway