A zero-day (also known as zero-hour or 0-day or day zero) vulnerability is an undisclosed computer-software vulnerability that hackers can exploit to adversely affect computer programs, data, additional computers or a network.
It is known as a "zero-day" because it is not publicly reported or announced before becoming active, leaving the software's author with zero days in which to create patches or advise workarounds to mitigate its actions. In effect, zero time has passed since the exploitable bug's existence was disclosed. Similarly, an exploitable bug that has been known for thirty days is sometimes called a 30-day exploit. The fewer the days the bug has been known about, the higher the chances that it has no fix or mitigation.
Source: Wikipedia, July 2017