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

That’s bad enough, but the agent also launched with a zero-day vulnerability that, luckily, has been patched. — Alan Henry, PC Magazine, 2 Oct. 2026

Election interference, ransomware, supply chain attacks and zero-day vulnerabilities in critical software systems all threaten national security. — Paul M. Nakasone, Washington Post, 1 Oct. 2026

On many occasions, these vulnerabilities were years-old, zero-day flaws that bad actors didn’t have the means to attack. — Ali El Kaafarani, Forbes.com, 1 Oct. 2026

The zero-day allows any app or terminal command to gain access to the token that authenticates users to their Muse account. — Dan Goodin, ArsTechnica, 21 Sep. 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