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

The exploit, which multiple researchers say works, is sending Microsoft scrambling, yet again, to patch a zero-day released by an anonymous researcher who has complained about the software maker’s handling of their bug reports. — Dan Goodin, ArsTechnica, 15 July 2026

Recent high-profile incidents such as the Anthropic Claude Mythos breach in early 2026 reveal how advanced AI models can autonomously map networks and exploit zero-day vulnerabilities at machine speed. — Anjana Susarla, Forbes.com, 9 July 2026

The next day was a zero-day, one of our few rest days of the trip. — John Todd, Outdoor Life, 9 July 2026

What sets it apart most is the zero-day waiting period. — Michael Kurko, USA Today, 28 June 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