ZERO-DAY ATTACK

It is also known as a zero hour attackattackattack, takes advantage of computer vulnerabilities that do not currently have a solution. Typically, a software company will discover a bug or problem with a piece of software after it has been released and will offer a patch — another piece of software meant to fix the original issue. A zero day attackattackattack will take advantage of that problem before a patch has been created. It is named zero day because it occurs before the first day the vulnerability is known.

From:wise geek!!