Zero-day exploits
Zero-day exploits are released on the same day the vulnerability - and, sometimes, the vendor patch - are released to the public. The term derives from the number of days between the public advisory and the release of the exploit. The term 'zero-day exploits' is sometimes (mis)used to indicate publicly known exploits for which no patches yet exist.
Zero-day protection is the ability to provide protection against zero-day exploits. Since zero-day attacks are generally unknown to the public, it is often difficult to defend against them. Zero-day attacks are often effective against secure networks and can remain undetected even after they are launched.
Many techniques exist to limit the effectiveness of zero-day memory corruption vulnerabilities, or buffer overflows. These protection mechanisms exist in contemporary operating systems such as Sun Microsystems Solaris, Linux, Unix, and Unix-like environments. Versions of Microsoft Windows XP Service Pack 2 and later include limited protection against generic memory corruption vulnerabilities. Desktop and Server protection software also exists to mitigate zero-day buffer overflow vulnerabilities. Typically these technologies involve heuristic termination analysis -- stopping them before they cause any harm.
These informations were extracted from the Wikipedia article about zero-day exploits.