Zero-Day Meaning

(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

Example: 2003, John Viega, Matt Messier, Secure Programming Cookbook for C and C++
  If your software is popular and has a high demand, you will want to defend against the "zero-day" cracker.
2014, Michael Gregg, CASP CompTIA Advanced Security Practitioner Study Guide: Exam CAS-002
  Before discussing ways to counter zero-day attacks, let's begin with the definition of what a zero-day attack is. A zero-day attack is one that the vendor does not yet know about or hasn't been fixed.
2005, Valdes et al, Recent Advances in Intrusion Detection
  Automatically creating reliable signatures of zero-day exploits is the focus of intense research efforts.
Used other than as an idiom: 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.