A zero-day attack or threat is a computer threat that tries to exploit unknown, undisclosed or unpatched computer application vulnerabilities. The term Zero Day is also used to describe unknown or Zero day viruses.
Zero-day exploits are released before the vendor patch is released to the public. Zero-day exploits generally circulate through the ranks of attackers until finally being released on public forums. The term derives from the age of the exploit. A zero-day exploit is usually unknown to the public and to the product vendor.