Frequency-division Multiplexing (FDM): A technique used in the telecommunications field that allows multiple signals to be transmitted simultaneously over a single communication line or medium by allocating a unique frequency range to each signal. This separation of the total bandwidth into various non-overlapping frequency sub-bands reduces interference and enhances communication reliability. FDM finds wide usage in radio and TV broadcasting, satellite and cable systems, as well as telephone networks.

Functional Testing: A type of software testing that validates the software system against the functional requirements/specifications. The purpose of functional tests is to test each function of the software application by providing appropriate input and verifying the output against the Functional requirements. This testing mainly involves black box testing and is not concerned about the source code of the application. It includes testing of user commands, data manipulation, searches, business processes, user screens, and integrations. Functional testing ensures that the application is behaving according to the intended functionality.

Fuzzing or Fuzz Testing: A software testing technique that involves providing invalid, unexpected, or random data as input to a computer program. The program is then monitored for exceptions such as crashes, failing built-in code assertions, or potential memory leaks. A fuzzer is a tool used to carry out fuzz testing. Fuzzing is often automated and is used to discover coding errors and security loopholes within software, operating systems, or networks. It is particularly useful for finding security concerns that might be exploited by malicious users.

Fail-open: A fail-open design refers to a security system's configuration that, in the event of a system failure, defaults to an "open" state, allowing all traffic or access. This approach prioritizes availability and continuity of operations over strict access control, often used in scenarios where service disruption is unacceptable despite the potential security risks it could introduce. This could also be doors that fail-open in case of an emergency, to allow for safe egress from a building.

Fail-Over (Fail-over): A backup operational mode in which the functions of a system component (such as a processor, server, network, or database) are assumed by secondary system components when the primary component becomes unavailable through either failure or scheduled downtime. Fail-over helps maintain high availability and reliability by ensuring that services continue to operate in the event of a component failure. This process can occur automatically without human intervention or can be manually triggered.

Failsafe: A fail-safe design ensures that in the event of a system failure or malfunction, the system will default to a safe or minimal risk condition to prevent or minimize harm or damage. Failsafe mechanisms are critical in various industries and technologies, such as in aviation, automotive safety systems, and nuclear power plants, where a failure could have severe consequences. In computing, a failsafe may involve switching to a backup system or mode of operation that maintains service continuity at a reduced capacity.