- A type of tunnel diode with reverse conduction characteristics, and which is well suited for use at microwave frequencies. back door A hidden software or hardware mechanism that allows access to a program, system, or network in a manner which completely eludes all security measures. It can be activated, for instance, by a seemingly random key sequence, and is usually left in place by the software developer. It is intended to allow for functions such as maintenance, and is a significant security risk when known by an unintended user or program. Also spelled backdoor. Also known as trapdoor.