Page "Computer insecurity" Paragraph 10

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 such as TEMPEST.