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.
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" …