Computer security:

Eavesdropping is the act of surreptitiously listening to a private conversation, typically between hosts on a network. For instance, programs such as Carnivore and NarusInsight have been used by the FBI and NSA to eavesdrop on the systems of internet service providers. Even machines that operate as a closed system (i.e., with no contact to the outside world) can be eavesdropped upon via monitoring the faint electro-magnetic transmissions generated by the hardware; TEMPEST is a specification by the NSA referring to these attacks.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". Eavesdropping on a closed system via the faint electro-magnetic trasmissions generated by the hardware has been referred to as what by the NSA?
TEMPEST