What does "zero-day" mean?

A vulnerability that has been discovered recently, and is yet unpatched or unmitigated; a zero-day vulnerability.

Zero-day

adj, noun

adj, noun ·Rare ·Advanced level

Noun

1
A vulnerability that has been discovered recently, and is yet unpatched or unmitigated; a zero-day vulnerability. idiomatic
"New Internet Explorer zero-day exploited in Hong Kong attacks"

Adjective

1
Of warez, released less than a day after the official product release. not-comparable
"Back then, having "zero day warez" (brand new cracked games that are not even one day old yet) on your BBS first was a big status symbol."
2
Of a vulnerability, newly discovered, and therefore still not fixed and possibly exploited by hackers or other criminals. not-comparable
3
Of an exploit or its threat or an attack, benefiting from a newly found and yet unpatched or unmitigated flaw in software or hardware; using a zero-day vulnerability. not-comparable
"If your software is popular and has a high demand, you will want to defend against the "zero-day" cracker."
4
Used other than figuratively or idiomatically: see zero, day. not-comparable
"Average residues of 1.44 and 2.18 mg/kg chlortetracycline were seen in liver and kidney, respectively, at zero-day withdrawal."

"Back then, having "zero day warez" (brand new cracked games that are not even one day old yet) on your BBS first was a big status symbol."

Data sourced from Wiktionary, WordNet, CMU, and other open linguistic databases. Updated March 2026.