Fail secure: A security term describing a feature or state where, upon failure, a system remains secure. In a fail-secure setup, if a component or system fails, it defaults to a locked or protected state rather than an open or unsecured one. This concept is commonly applied to access control systems, such as locked doors that remain locked during a power outage, ensuring that security is maintained even when the system's primary operation is compromised.

Fail-soft: A type of fail-over system that is designed to provide a reduced level of service in the event of a failure or interruption. It is used to ensure the availability of services in case of failure but at a lower level than the primary system. For example, a fail-soft system may provide a limited number of services in case of a failure rather than all services.

Fallback Procedures: Predetermined plans designed to help an organization maintain or quickly resume mission-critical functions in the event of an interruption or failure. The fallback procedures come into play when normal processes cannot continue and often involve the use of redundant systems, backups, or alternative methods to continue operations until normal conditions are restored.

Fall-through Logic: In programming, fall-through logic occurs in switch-case statements where the absence of a break statement causes the execution to continue from one case into the subsequent case. It's intentional in certain programming scenarios but should be used cautiously to prevent logic errors. In security, careful design is required to avoid unintended actions from fall-through behavior, especially in authentication or access control decisions.

False Acceptance Rate (FAR): A measure used in biometric security systems to indicate the probability that the system will incorrectly accept an unauthorized user. It is one of the key metrics, along with the False Rejection Rate (FRR), used to assess the performance of a biometric system. A low FAR is desired as it reflects a higher level of security, indicating that the system is less likely to permit access to an unauthorized individual. However, system designers must balance FAR with FRR to ensure the system is both secure and user-friendly.

False Negative: An instance in system monitoring or threat detection where a genuine problem, such as a security breach, system failure, or malicious activity, goes unnoticed. A false negative essentially means the system failed to recognize and alert a real issue, which is highly concerning as it can lead to significant damage or security compromises without immediate detection or resolution.