n. (Physics) The specific absorption and re-emission of electromagnetic radiation at characteristic wavelengths by atomic nuclei in a magnetic field. It is abbreviated NMR. The wavelength of the radiation absorbed depends on the type of nucleus, the intensity of the magnetic field, and the local chemical environment in which the nucleus resides. It is the latter effect (called the chemical shift), by which atoms of specific elements in different chemical compounds show a different resonance frequency, which gives rise to the greatest utility of this phenomenon in analyzing the chemical structure of substances. Similar effects of the chemical environment permit the discrimination of different types of living tissue by virtue of their different chemical composition, thus permitting utilization of the phenomenon in medical diagnostic instruments, especially for magnetic resonance imaging. [ PJC ]