1. A resonant absorption of electromagnetic radiation by a paramagnetic substance, having unpaired electrons, when the energy levels are split by the application of a strong magnetic field.
2. Magnetic resonance arising from the magnetic moment of unpaired electrons in a paramagnetic substance or in a paramagnetic center in a diamagnetic substance (a group of substances which in a magnetic field become magnetic in an opposite direction to that of iron or relating to a substance that is repelled by a magnet).