Measurement and signature intelligence, or MASINT, is the discipline of detecting and identifying a target from the physical signatures it emits: radar returns, infrared and thermal emissions, acoustic and seismic patterns, or chemical traces. Where GEOINT shows what something looks like and SIGINT captures its communications, MASINT characterises what it is from the measurable data it gives off.
MASINT is the most technical of the classic intelligence disciplines and is largely the domain of specialised sensors and agencies. It rarely appears directly in open-source work, but the idea matters: publicly available sensor feeds — seismic networks, environmental and weather monitors, flight and maritime signals — carry measurable signatures a careful analyst can read to corroborate an event.