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 Military & Associated Terms)
Signals Intelligence (SIGINT) refers to electronic transmissions that can be collected by ships, planes, ground sites, or satellites. Communications Intelligence (COMINT) is a type of SIGINT and refers to the interception of communications between two parties. U.S. SIGINT satellites are designed and built by the National Reconnaissance Office, although conducting U.S. signals intelligence activities is primarily the responsibility of the National Security Agency (NSA).
https://www.fbi.gov/about/leadership-and-structure/intelligence-branch
Friedersdorf, C. (2014, June 16). The limits of signals intelligence. The Atlantic. Link to source
House of Representatives Permanent Select Committee on Intelligence, 104th Cong. SIGINT: Signals Intelligence (Comm. Print 1995). Link to source
National Security Agency/Central Security Service, Signals Intelligence Directorate. (2011). United States signal intelligence directive (USSID SP0018). Link to source
The White House, Office of the Press Secretary. (2014, January 17). Remarks by the President on review of signals intelligence. Link to source
Richelson, J. T. (2015, March 15). The CIA and signals intelligence. The National Security Archive, George Washington University. Link to source
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)
ELINT: The Interception and Analysis of Radar Signals
ONLINE RESOURCES
Bernard, R. L. (2009). Electronic intelligence (ELINT) at NSA. Center for Cryptologic History, National Security Agency. Link to source
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 Communications Intelligence (COMINT) as "technical information and intelligence derived from foreign communications by other than the intended recipients"
(DOD Dictionary of Military and Associated Terms)
Strategic Intelligence Management
ONLINE RESOURCES
Center for Cryptologic History. (n.d.). COMINT stations overseas. National Security Agency Central Security Service. Link to source
Keller, J. (2012, September 5). Aircraft-based COMINT for military, border, and maritime use introduced by Rohde & Schwarz. Military & Aerospace Electronics. Link to source
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)