Answer Description
A zero-day attack is an attack that is still unknown to the vendor, software developer, and/or system administrators. It is an attack that is newly discovered by malicious users.
Wikipedia
A zero-day (also known as 0-day) is a computer-software vulnerability either unknown to those who should be interested in its mitigation (including the vendor of the target software) or known and a patch has not been developed Until the vulnerability is mitigated, hackers can exploit it to adversely affect programs, data, additional computers or a network An exploit directed at a zero-day is called a zero-day exploit, or zero-day attack The term "zero-day" originally referred to the number of days since a new piece of software was released to the public, so "zero-day software" was obtained by hacking into a developer's computer before release
Zero-day_(computing) - Wikipedia, the free encyclopedia