1. The act of listening, carrying out surveillance on, and/or recording the emissions of one's own or allied forces for the purpose of maintaining and improving procedural standards and security, or for reference, as applicable.
2. The act of listening, carrying out surveillance on, and/or recording of enemy emissions for intelligence purposes.
3. The act of detecting the presence of signals, such as electromagnetic radiation, sound, or visual signals, and the measurement thereof with appropriate measuring instruments.
4. The act of detecting the presence of radiation and the measurement thereof with radiation measuring instruments. Synonym radiological monitoring.
Source: from Federal Standard 1037C and from the Department of Defense Dictionary of Military and Associated Terms