Book excerpt: Zero day refers to software vulnerabilities or security flaws that are unknown to the software vendor and for which there is no available patch or solution. This means that the vulnerability can be exploited by attackers before it is discovered and fixed by the vendor. The term "zero day" comes from the fact that the vulnerability is known to attackers on the same day it is discovered, and before any patch or solution is available, leaving the software vulnerable to attacks. Zero day vulnerabilities are particularly dangerous because they can be used to launch targeted attacks against specific organizations or individuals, and can remain undetected for a long time. Attackers can use zero day exploits to gain unauthorized access to systems, steal sensitive information, or cause other types of damage. Organizations must reduce the risk of zero day vulnerabilities by keeping software up-to-date, applying security patches as soon as they become available, and monitoring for unusual activity that could indicate a zero day attack. Additionally, using security tools such as intrusion detection systems and endpoint protection can help detect and prevent zero day attacks.