Zero day or 0day refers to software, media, or information that is obtained either slightly prior to or on the day of the official release. Items gained further in advance are deemed "Negative day" or sometimes "-day".

When applied to software or media, zero day has connotations of illegality. Zero day software is warez, that is, illegal. The term derives from the day when the software is illegally available. Counting from zero, software that is illegally available on the day of its release is available on the zero^th day, hence the term. Similarly, one can refer to one day, two day, etc., software. One can also refer to negative day software for software that is illegally available before its official release, but such software may also be referred to as zero day.

When applied to information, zero day usually means information that is not publicly available. This is often used to describe security vulnerabilities exploits which are unknown to computer security professionals. These are, figuratively speaking, the system administrator's worst nightmare: Since the attack is completely unknown it is impossible to defend against, and consequently it may happen that one's entire network is taken over before one has a chance to respond.

Zero-day protection is the ability to provide protection against zero-day exploits. Typically this would be some sort of sniffer which would detect buffer overruns and other common exploits, stopping them before they actually caused any harm. However, a perfect solution of this kind is theoretically impossible, since it is impossible to analyze any arbitrary code to determine if it is malicious (such an analysis reduces to the Halting Problem).