A zero-day (also known as zero-hour or 0-day) vulnerability is a previously 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 once the flaw becomes known, the software's author has zero days in which to plan and advise any mitigation against its exploitation (for example, by advising workarounds or by issuing patches).