zero-day

Short definition from Wiktionary

Adjective

1. 1
   Of warez, released less than a day after the official product release.
2. 2
   Of a vulnerability, newly discovered, and therefore still not fixed and possibly exploited by hackers or other criminals.
3. 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. 4
   Used other than figuratively or idiomatically: see zero, day.
5. 5
   Alternative spelling of zero-day.

Noun

1. 1
   A vulnerability that has been discovered recently, and is yet unpatched or unmitigated; a zero-day vulnerability.
2. 2
   Alternative spelling of zero-day.