TEMPEST - technical definition

Refers to external electromagnetic radiation from data processing equipment and the security measures used to prevent them. Almost all electronic equipment emanates signals into free space or surrounding conductive objects such as metal cabinets, wires and pipes. Equipment and cables that meet TEMPEST requirements have extra shielding in order to keep data signals from escaping and being picked up by unauthorized listeners. It is also possible to use TEMPEST software that generates sufficient electronic noise to mask meaningful radio-frequency emissions.

TEMPEST was a code name for U.S. military operations throughout the 1960s. Later, it was turned into an acronym. Take your pick: Telecommunications Electronics Material Protected from Emanating Spurious Transmissions or Transient ElectroMagnetic Pulse Emanation STandard. See emanation and EMSEC.