Signals Intelligence (SIGINT) can be defined as a category of intelligence comprising either individually or in combination all communications intelligence (COMINT), electronic intelligence (ELINT), and foreign instrumentation signals intelligence (FISINT), however transmitted; or more simply as intelligence derived from communications, electronic, and foreign instrumentation signals (US Dept. of Defense Dictionary of Miilitary & Associated Terms).
Electronic Intelligence (ELINT) is intelligence gathered by the use of electronic sensors. In ELINT, intelligence gathered is generally those other than personal communications. (Dictionary.com).
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Communications Intelligence (COMINT) is information gathered from the communications of individuals by interception of wire or radio communications (Dictionary.com)
The U.S. Joint Chiefs of Staff define it as "technical information and intelligence derived from foreign communications by other than the intended recipients" (DOD Dictionary of Military and Associated Terms)
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Foreign Instrumentation Signals Intelligence (FISINT) is technical and intelligence information derived from intercept of foreign instrumentation signals which include, but are not limited to, telemetry, beaconry, electronic interrogators, tracking/fusing/arming command systems, and video data link (Military Periscope)
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