Definition of "zero-day" in English

Adjective

1. Of warez, released less than a day after the official product release.

2. Of a vulnerability, newly discovered, and therefore still not fixed and possibly exploited by hackers or other criminals.

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.

4. Used other than figuratively or idiomatically: see zero,‎ day.

   • Average residues of 1.44 and 2.18 mg/kg chlortetracycline were seen in liver and kidney, respectively, at zero-day withdrawal.

Noun

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

   • New Internet Explorer zero-day exploited in Hong Kong attacks
   • These days, however, more zero days are being used and discovered.
   • All the four zero-days originally were reported to Microsoft, affecting Internet Explorer on the desktop.