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, or unaddressed by, 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 In the jargon of computer security, "Day Zero" is the day on which the interested party (presumably the vendor of the targeted system) learns of the vulnerability, leading to the vulnerability being called a "zero-day"
Zero-day_(computing) - Wikipedia, the free encyclopedia