Adjective
Of warez, released less than a day after the official product release.
Of a vulnerability, newly discovered, and therefore still not fixed and possibly exploited by hackers or other criminals.
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.
Used other than figuratively or idiomatically: see zero, day.
Noun
A vulnerability that has been discovered recently, and is yet unpatched or unmitigated; a zero-day vulnerability.