Signals Intelligence (SIGINT) is a type of intelligence that involves the collection and analysis of signals, such as electronic communications, radar signals, and other transmissions. This information can be used to provide insight into an adversary's capabilities, intentions, and activities. SIGINT can be used in a variety of ways, including monitoring foreign military activities, supporting counterintelligence operations, and informing policy and military decision making.
Source: Ettenberg, S. (2015). Signals intelligence (SIGINT) in modern conflict. Journal of Strategic Security, 8(2), 1-15.