What BEST describes a zero-day vulnerability?

A zero-day attack or exploit takes place without the knowledge of the vendor. The other responses are not an accurate description of a zero-day attack. The fact that a system has not been breached yet does not necessarily mean it is a zero-day, and a zero-day attack may not always be easily exploitable. Additionally, a zero-day attack does not indicate the duration for which the attacker has access to the system.