Electronic Intelligence a.k.a. Elint
Electronic intelligence is the act of gathering information about someone or something through the use of any electronic measure, an example being listening in on a radio conversation.
Electronic Intelligence in the military is usually gathered by Elint aircraft. Elint aircraft fly as close to an enemy's defenses as it can, and simply listens in. Occasionally, in a wartime situation, Elint aircraft can actually mislead the enemy by broadcasting static interference or by pretending to be a commanding general, and giving out misleading orders.
Examples of Elint aircraft are the Lockheed P-3 Orion, the Lockheed U-2, The Boeing RC-135, the British Aerospace Canberra, the British Aerospace Nimrod, Ilyushinn Il-20 Coot-A, and the Ilyushin Il-38 May, along with many others.
Also, see Signal Intelligence, or Sigint.