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) vulnerability is a computer-software vulnerability that is unknown to those who should be interested in mitigating the vulnerability (including the vendor of the target software) Until the vulnerability is mitigated, hackers can exploit it to adversely affect computer 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 software that had been obtained by hacking into a developer's computer before release
Zero-day_(computing) - Wikipedia, the free encyclopedia