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Refers to investigations and studies of compromising emanations. Compromising emanations are unintentional intelligence bearing signals which, if intercepted and analyzed, may disclose the information transmitted, received, handled, or otherwise processed by any information-processing equipment. These emanations consist of electrical or acoustical energy emitted by any number of sources. 
The term TEMPEST is often used for the entire field of Emission Security or Emanations Security (EMSEC).
The US and NATO TEMPEST standards define three levels of protection requirements.  TEMPEST certification must apply to entire systems, not just to individual components.