adjective: (computing, idiomatic) (of vulnerability) newly discovered, and therefore still not fixed and possibly exploited by hackers or other criminals
adjective: (computing, idiomatic) (of exploit, its threat, or attack) benefiting from newly found and yet unpatched or unmitigated flaw in software or hardware; using zero-day vulnerability
adjective: Used other than figuratively or idiomatically: see zero, day.
noun: (computing, idiomatic) vulnerability that has been discovered recently, and is yet unpatched or unmitigated; zero-day vulnerability